NUVO Surgical Lighting Technology

The Basics

Color Temperature

Color Rendering Index (CRI)

Units of Measure Defined

Heat to Light Ratio

International Standard IEC 60601-2-41

The full range of energy that is emitted from small to large wavelengths is known as the electromagnetic spectrum.

Everyone at one time has listened to the radio. A radio is nothing more then a device that receives electromagnetic radiation (radio waves) and then processes the information so that the brain interprets the output as sound. When a specific radio wave is transmitted, the distance from the peak of the first wave to the peak of the second wave is known as the wavelength for that specific radio wave. The wavelength of radio waves is in the range of 10^-1 to 10³ plus meters as shown in Figure 1.

Figure 1: Lightwaves as Part of the Electromagnetic Spectrum^1,2,3

Lightwaves are also part of the electromagnetic spectrum. They exist in the approximate range of 380 x 10^-9 meters to 740 x 10^-9 meters. 10^-9 meters is defined as a nanometer, abbreviated nm. Thus, lightwaves exist in the approximate range of 380 nm to 740 nm (the visible spectrum).

Since lightwaves travel in space, the number of lightwaves that pass a given point in any one second is defined as frequency, better known as cycles per second. One cycle per second is defined as one hertz, abbreviated Hz. 1012 cycles per second is known as a tera hertz, abbreviated THz. Therefore, it can be seen from Figure 1, that the wavelength of any lightwave is very small and that a large number of lightwaves pass a given point in any one second. It can also be seen that visible light makes up a very small portion of the electromagnetic spectrum.

Like a radio, the human eye can be thought of as a device that receives electromagnetic radiation. The electromagnetic radiation that the eye receives is reflected from objects in the range of 380 nm to 740 nm (the visible spectrum) and sent to the brain via the optic nerve for interpretation as color.

The retina, located at the back of the eye, contains millions of rods and cones. Rods process low levels of light energy for night vision. Cones process light energy, primarily during daylight hours, for color vision.

There are three groups of cones that are stimulated by energy within the visible spectrum. One group of cones is sensitive to red light (long wavelengths), another group is sensitive to green light (middle wavelengths), and yet another group is sensitive to blue light (short wavelengths). Figure 2 shows the location of the retina, rods, and cones within the human eye.

Figure 2: Cross-Section of the Eye Showing the Lens, Retina, Rods, Cones, and Fovea, etc.⁴

Opposite the lens is an area within the retina called the fovea. It has the highest concentration of cones. When light rays focus on the fovea by way of the lens the viewer can discern fine detail and has very good forward vision.

Over the years, science has shown that there are three primary colors, red, green, and blue. These colors are called primary colors because when mixed together in the right proportions all other colors visible to the eye can be created. For example, red added to blue creates magenta, red added to green creates yellow, and green added to blue creates cyan. Red, blue, and green added together creates white. This concept of additive colors is illustrated in Figure 3.

Figure 3: Combining Red, Blue, and Green to Create White2,5

CIE (Commission Internationale d’Eclairage):

Based upon human perception studies, it was concluded that one persons color perception may be different then another persons color perception. Consequently, in 1931, the International Commission on Illumination (Commission Internationale de l'Eclairage, or CIE) developed a concept called the "standard observer". The "standard observer" is based upon experimental human color perception averages from small numbers of people considered to have normal color vision. Because of the small human population sample size used to define the "standard observer", it is unlikely that a single person would see colors exactly the same as defined by the average of the standard observer data (color perception becomes totally subjective). Data collected from experiments leading to the 1931 "standard observer" definition used only the fovea of the eye (Ref. Figure 2) which covers only about a 2-degree field of vision. A 2-degree field of vision is approximately equivalent to an individual holding a penny at arms length and focusing on the penny with the eyes.

Using the 2-degree standard observer data and knowing that all visual colors can be created using combinations of red, green, and blue, CIE created an x-y plot (chromaticity diagram) of the visual colors. This plot is known as the 1931 2-degree standard observer CIE chromaticity diagram shown in Figure 4.

Figure 4: 1931 2-Degree Standard Observer CIE Chromaticity Diagram6,7

It must be emphasized that Figure 4 is just an approximation of visual colors since it is difficult to plot all known colors visible to the eye. It is even more difficult for a color printer to print all known colors visible to the eye.

The oval in Figure 4 is considered to be in the "white" area of vision as perceived by the human eye. Notice that the oval containing the "white" area of vision ranges from what appears to be a yellow-red region to a very white region to a bluish region. Therefore, from Figure 4, one can conclude that there are many degrees of "whiteness".

Along with the chromaticity diagram shown in Figure 4, two other parameters are used to characterize color. These are color temperature or correlated color temperature and color rendering index (CRI).

Color temperature or correlated color temperature (CCT) is the perception of how cool or warm the light from a surgical luminaire appears to the eye. Light from luminaires with a low color temperature, such as ALM, appears red. Light from luminaires with higher color temperatures, such as Berchtold and NUV, appears more white or blue, depending on how high the color temperature may be.

Color temperature or correlated color temperature is expressed in the Kelvin temperature scale. The relationship between the Kelvin temperature scale and the Fahrenheit temperature scale is as follows:

An analogy of correlating a temperature scale to color is the heating of a steel bar. When heat is initially applied to the bar, the bar begins to glow a dull red. As more and more heat is applied and the temperature of the bar increases, the color of the heated area changes from a dull red to a bright red to white and finally blue-white. Temperature is measured in degrees Kelvin and correlated to the color. Physicists use a defined protocol to correlate the temperature, in degrees Kelvin, of a heated substance to the color of the heated substance as perceived by the human eye.

Color rendering index (CRI) is a measure of the color shift objects undergo when illuminated by a light source as compared with the color of those same objects when illuminated by a reference source, tungsten lamps or daylight, at the same color temperature.¹⁰ The color rendering index of a surgical luminaire is calculated according to the rules described in CIE publication 13.3-1995. A good quality surgical luminaire will have a CRI between 85 and 100. Note: CRI of the reference source is assigned a value of 100.

Figure 5 shows the color temperature of various sources in degrees Kelvin. Lamp CRI values are also shown.

Figure 5:⁹ Color Temperature Comparison of Natural Daylight and Various Lamp Sources

From the 2-degree standard observer data, scientists have also concluded that the human eyes perception of "brightness" is different for each wavelength in the visible portion of the electromagnetic spectrum. For example, one watt of power at a wavelength of 555 nanometers (green portion of the visible spectrum, reference Figure 1) is perceived as approximately five times brighter then one watt of power at a wavelength of 637 nanometers (red portion of the visible spectrum, reference Figure 1). The same is true for light at a wavelength of 489 nanometers (blue portion of the visible spectrum, reference Figure 1). One watt of power at a wavelength of 555 nanometers is perceived as approximately five times brighter then one watt of power at a wavelength of 489 nanometers. In fact, the eye perceives the monochromatic wavelength of 555 nanometers as being most bright in the visible spectrum when the same input power is applied at each wavelength throughout the visible spectrum. In essence the human eyes perception of brightness, for the same input power at each wavelength, is approximately zero at 380 nanometers, peaks at 555 nanometers, and is approximately zero again at 740 nanometers.

The concept of "brightness" is then derived by comparing the human eyes response to one watt of power at a monochromatic wavelength at 555 nanometers to one watt of power at any other wavelength. A comparison curve, known as "relative luminosity", can be generated with the value of 1 assigned to the 555 nanometer wavelength. Since the response of the human eye is compared to the eyes response at 555 nanometers, values less then one are derived for all other wavelengths throughout the visible region. When the brightness response of the eye is studied under daylight conditions (cones are primarily used to process light energy during daylight hours), the comparison curve or relative luminosity is defined as the photopic response curve. The symbol V(λ) is used to identify the photopic response.

Using the "brightness" study results from the 2-degree standard observer data, physicists have defined the lumen as the amount of monochromatic radiation whose frequency is 540 x 10¹²Hz and whose power is 1/683 Watt.¹¹ In other words, 1 Watt of monochromatic radiation at 555 nanometers provides an amount of light equal to 683 lumens.

A lumen per square foot is defined as a footcandle. A lumen per square meter is defined as a lux. There are approximately 10.76 lux per footcandle.

Figure 6 illustrates the photopic response of the human eye. In Figure 6, the primary ordinate axis (left vertical axis) shows the relative luminosity as a function of wavelength, while the secondary ordinate axis (right vertical axis) shows the number of lumens per watt of input power as a function of wavelength.

Some important things to note about the photopic response curve shown in Figure 6:

The peak response occurs at 555 nanometers. For 1 watt of power at 555 nanometers 683 lumens are available.
The curve is smooth and continuous.
All of the energy is completely within the visible region of the electromagnetic spectrum.

Another important parameter to take into consideration for surgical lighting systems is known as heat to light ratio. Heat to Light Ratio is a measure of energy throughout all wavelengths of the electromagnetic spectrum for which energy is emitted by the surgical lighthead divided by the energy emitted in the visible region of the spectrum by the surgical lighthead.

A low heat to light ratio means that the energy emitted by a surgical luminaire is efficiently being used within the visible portion of the electromagnetic spectrum to evoke the sensation of brightness at the surgical site. Consequently, the end-user of the surgical light may perceive the pattern at the surgical site as being very cool and bright. Conversely, a high heat to light ratio means that the energy emitted by a surgical luminaire is inefficiently being used within the visible portion of the electromagnetic spectrum to evoke the sensation of brightness at the surgical site. Consequently, the end-user of the surgical light may perceive the pattern at the surgical site as being very warm or hot, even though it may be bright.

NUV measures Heat to Light Ratio in terms of mircoWatts per centimeter squared per footcandle. From NUV’s measurements, the NUV light’s Heat to Light Ratio in the 300-1100 nanometer range is approximately 3.71 microWatts per centimeter squared per footcandle or

. NUV’s heat to light ratio measurement of Berchtold’s 571C lighting fixture, in the 300-1100 nanometer range, is

. NUV measured the heat to light ratio of Berchtold’s 650C lighting fixture, in the 300-1100 nanometer range, as

From these values, it can be seen that the NUV light has less energy at the surgical site then either of the Berchtold lighting fixtures for the same number of footcandles delivered to the surgical site. Therefore, the end-user may perceive the NUV light as being just as bright as the Berchtold lights but much cooler then the Berchtold lights.

Another advantage to less energy at the surgical site is the need to irrigate the site to keep tissue from drying out. In essence, a lower heat to light ratio suggests that the surgical team needs to irrigate the surgical site less often when using the NUV light compared to using the Berchtold light for the same operative procedure when illuminated at the same intensity.

Figure 6: Photopic Response of the Human Eye

Relationship between CRI and Heat to Light Ratio:

Color Rendering Index (CRI) is determined by the way energy is distributed within the visible region of the electromagnetic spectrum. Heat to Light Ratio is determined from the efficient use of energy emitted by a surgical lighthead to evoke the sensation of brightness at the surgical site.

Figure 7 illustrates the energy distribution (spectral power distribution or SPD) of a curve that has the same shape as the luminosity or photopic curve shown in Figure 6.

Figure 7: Spectral Power Distribution for a Curve with the Same Shape as the Luminosity Curve shown in Fig. 6

Some important things to note about the Spectral Power Distribution Curve shown in Figure 7:

The peak response occurs at 555 nanometers.
The curve is smooth and continuous.
All of the energy is completely within the visible region of the electromagnetic spectrum. Therefore, no energy is emitted outside the visible region of the electromagnetic spectrum. Consequently, one would expect a relatively low heat to light ratio.
Because the curve is a spectral power distribution curve, the units of the ordinate or y-axis are Watts per meter squared per nanometer.
The shape of the curve within the visible spectrum determines the Color Rendering Index. The color rendering index is calculated according to the rules described in CIE publication 13.3-1995.

From the spectral power distribution curve shown in Figure 7, correlated color temperature, CRI, and heat to light ratio are calculated. The results are shown in Table 1.

Correlated Color Temperature °K	Color Rendering Index, CRI	Heat to Light Ratio from 300-1100 nm
4410	42

Table 1: Correlated Color Temperature, CRI, and Heat to Light Ratio for the Spectral Power Distribution Curve shown in Figure 7

Even though the heat to light ratio has a relatively low value of 2.18 microWatts per centimeter squared per footcandle, the CRI value of 42 is also low. As stated on page 4, a good quality surgical luminaire will have a CRI between 85 and 100. Therefore, to increase the CRI value, the shape of the spectral power curve shown in Figure 7 must be changed. Energy must also be added to the visible region of the spectrum.

Figure 8 shows a theoretical spectral power distribution curve superimposed over the spectral power distribution curve shown in Figure 7. The theoretical SPD curve is shown in blue in Figure 8.

Figure 8: Theoretical Spectral Power Distribution Curve Superimposed over the SPD Curve shown in Figure 7

Some important things to note about the theoretical Spectral Power Distribution Curve (blue curve) shown in Figure 8:

Energy is added to the blue-green region of the visible spectrum from about 380 nanometers to approximately 532 nanometers.
The peak response occurs at approximately 600 nanometers instead of 555 nanometers.
Energy is added to the orange-red region of the visible spectrum from about 600 nanometers to approximately 740 nanometers.
Most of the energy is completely within the visible region of the electromagnetic spectrum.
The shape of the curve within the visible spectrum determines the Color Rendering Index. The color rendering index is calculated according to the rules described in CIE publication 13.3-1995.

From the spectral power distribution curve shown in Figure 8, correlated color temperature, CRI, and heat to light ratio are calculated. The results are shown in Table 2.

Correlated Color Temperature °K	Color Rendering Index, CRI	Heat to Light Ratio from 300-1100 nm
4250	92

Table 2: Correlated Color Temperature, CRI, and Heat to Light Ratio for the Theoretical Spectral Power Distribution Curve (blue curve) shown in Figure 8

As can be seen from Table 2, increasing CRI increases heat to light ratio. Therefore a tradeoff is made between CRI and heat to light ratio.

Early in the design process, the NUV design team decided, supported by focus panel discussion, that it was more important to minimize energy at the surgical site, thus giving the perception of a cooler beam of light, then having a light with a high CRI value. As shown, a relatively high CRI value delivers more energy to the surgical site, thus giving the perception of a hotter beam of light. A low heat to light ratio minimizes energy at the surgical site whereas a high heat to light ratio increases energy at the surgical site for the same level of illumination or brightness.

The NUV CRI value of 85 puts the NUV light in the range of a good quality surgical luminaire by minimizing energy at the surgical site with a heat to light ratio, in the 300 to 1100 nanometer range, of approximately 3.71 microWatts per centimeter squared per footcandle. This compares to a Berchtold CRI value of approximately 92 with the Berchtold lighting fixtures having a heat to light ratio of approximately 4.40 microWatts per centimeter squared per footcandle in the 300 to 1100 nanometer range. Because of the higher heat to light ratio, the Berchtold surgical lighting fixtures are perceived as being hotter then the NUV surgical lighting fixture with the same level of illumination at the surgical site. Note: The Berchtold heat to light ratio is approximately 19% higher then the NUV heat to light ratio. Therefore, Berchtold is putting about 19% more energy at the surgical site then NUV for the same illumination level.

IEC 60601-2-41 is an international standard that defines particular requirements for the safety of surgical luminaires and luminaires for diagnosis. The FDA references IEC 60601-2-41 in its "Guidance Document for Surgical Lamp 510(k)s" and encourages manufacturers to follow the IEC standard.

It should be noted that the FDA classifies surgical luminaires as a Class II device.

IEC 60601-2-41 specifies the chromaticity coordinates for surgical luminaires using the 1931 2-degree standard observer CIE chromaticity diagram. Figure 9 superimposes the color box defined by IEC chromaticity coordinates specified for surgical luminaries over the 1931 2-degree standard observer CIE chromaticity diagram shown in Figure 4. The IEC color box is shown as a bright green in Figure 9.

Figure 9: Surgical Luminaire Color Box Defined by IEC 60601-2-41⁸

The surgical luminaire color box shown in Figure 9 may seem small compared to the overall range of colors shown in the CIE chromaticity diagram. However, the human eye has keen enough vision to discern color differences from surgical luminaires emitting light from different sides of the IEC color box, particularly when the luminaires are placed side by side.

Because of the human eye’s ability to discern these color differences, NUV has designed a much smaller color box for the NUV light. The NUV light color box falls within the color box specified by IEC 60601-2-41. Because of the smaller NUV color box, the end-user of the NUV surgical light is unable to discern color differences between two or more lightheads when the lightheads are operated side by side in the number 5 intensity position. Figure 10 superimposes the NUV color box over the IEC color box shown in Figure 9. In Figure 10, the IEC color box is again shown as bright green while the NUV color box is shown in light blue.

As can be seen in Figure 10, the NUV color box is much smaller then the IEC color box and is very small compared to the overall range of colors visible to the human eye.

Figure 10: NUV Color Box Relative to IEC 60601-2-41 Color Box shown in Figure 9

Table 3 shows the classification of surgical luminaires and luminaires for diagnosis by IEC 60601-2-41. The NUV light is classified, by its intended use, as a major surgical luminaire.

Some important things to note about the information presented in Table 3.

Testing a surgical luminaire to ensure compliance to the requirements of Table 3 is conducted at the rated voltage of the luminaire.
The maximum intensity or central illuminance (E_c) is 40 klx ≤ E_c ≤ 160 klx or approximately between 3,700 footcandles and 14,870 footcandles.
Color Temperature or Correlated Color Temperature is 3,000 degrees Kelvin minimum and 6,700 degrees Kelvin maximum.
Color Rendering Index is 85 minimum and 100 maximum.
Pattern diameter is determined where the illuminance reaches 10% of the Peak Intensity or Central Illuminance, E_c. The notation used by IEC for a pattern diameter of 10% of the Peak Intensity is d₁₀. The notation used by IEC for a pattern diameter of 50% of the Peak Intensity is d₅₀. Note: IEC requires that the pattern diameter at 50% of the peak intensity be greater then or equal to the pattern diameter at 10% of the peak intensity. Reference Figure 11.

The NUVO light satisfies all requirements of IEC 60601-2-41.

Requirements	Clause	Type of luminaire
		Luminaires for diagnosis	Surgical luminaires
		Luminaires for diagnosis	Minor (treatment)	Major and system
EQUIPMENT classification	14.2 a)2)	No requirement	Class I, or Class II with connector to PA^a	Class I, or Class II with connector to PA^a
Fail Safe	2.10.101	No	No	Yes
Anesthesia (intended purpose)		Localized	Local/general	Local/general
Intended location		Examination room	Operating room	Operating room
Sterile handle (standard)		No	Yes	Yes
Central illuinance (E_c)	50.102.1.1 a)	No requirement	40 klx ≤ E_c ≤ 160 klx	40 klx ≤ E_c ≤ 160 klx
Light field diameter (d₁₀)	50.102.1.1 b)	No requirement	Yes^b	Yes^b
Light distribution	50.102.1.1 b)	No requirement	Yes^c	Yes^c
Shadow dilution	50.102.1.1 c)	No requirement	Yes^d	Yes^d
Colour temperature	50.102.2.1	3000 K ≤ T_c ≤ 6700 K	3000 K ≤ T_c ≤ 6700 K	3000 K ≤ T_c ≤ 6700 K
Colour rendering index	50.102.2.1	85 ≤ P_a ≤ 100	85 ≤ P_a ≤ 100	85 ≤ P_a ≤ 100
Maximum value for total irradiance E_e	50.102.3.1	Yes^e	Yes^e	Yes^e
a PA means potential equalization conductor. b LIGHT FIELD DIAMETER (d₁₀) where the illuminance reaches 10% of CENTRAL ILLUMINANCE E_c. c Diameter (d₅₀) where the illuminance reaches 50% of CENTRAL ILLUMINANCE E_c. d Percentage of remaining illuminance when the beam is obstructed by one or two masks, with or without tube. e Information on the total irradiance E_c for the given CENTRAL ILLUMINANCE E_c.

Table 3: Classification of Surgical Luminaires and Luminaires for Diagnosis, from IEC 60601-2-41

Figure 11: Theoretical Light Distribution for Three Different Surgical Lightheads

Figure 11 illustrates a theoretical light distribution for three different surgical lightheads. All three lightheads have a peak intensity of 10,000 footcandles and a 10% pattern size of 10 inches.

The light distribution shown in blue satisfies the minimum pattern requirements of IEC, i.e., the pattern size at 50% of the peak intensity must be greater then or equal to the pattern diameter at 10% of the peak intensity. Since the peak intensity of the blue curve is 10,000 footcandles, 50% of the peak intensity is 5,000 footcandles. 10% of the peak intensity is 1,000 footcandles. The pattern at 50% of the peak intensity, for the blue curve, d₅₀, ranges from -2.5 inches to 2.5 inches for a d₅₀ pattern diameter of 5 inches. The pattern at 10% of the peak intensity, d10, ranges from -5 inches to 5 inches for a d₁₀ pattern diameter of 10 inches. The IEC requirement is d₅₀ ≥ 0.5 x d₁₀. Substituting the pattern size values yields: 5 ≥ 0.5 x 10. As can be seen, the minimum IEC requirement is satisfied. One half of 10 is equal to 5.

The light distribution shown in red does not satisfy the minimum pattern requirements of IEC, i.e., the pattern size at 50% of the peak intensity must be greater then or equal to the pattern diameter at 10% of the peak intensity. Since the peak intensity of the red curve is 10,000 footcandles, 50% of the peak intensity is 5,000 footcandles. 10% of the peak intensity is 1,000 footcandles. The pattern at 50% of the peak intensity, for the red curve, d₅₀, ranges from approximately -1.65 inches to approximately 1.65 inches for a d₅₀ pattern diameter of approximately 3.3 inches. The pattern at 10% of the peak intensity, d₁₀, ranges from -5 inches to 5 inches for a pattern diameter of 10 inches. The IEC requirement is d₅₀ ≥ 0.5 x d₁₀. Substituting the pattern size values yields: 3.3 ≥ 0.5 x 10. As can be seen, the minimum IEC requirement is not satisfied. 3.3 inches is less then one half of 10.

The light distribution shown in green does satisfy the minimum pattern requirements of IEC, i.e., the pattern size at 50% of the peak intensity must be greater then or equal to the pattern diameter at 10% of the peak intensity. Since the peak intensity of the green curve is 10,000 footcandles, 50% of the peak intensity is 5,000 footcandles. 10% of the peak intensity is 1,000 footcandles. The pattern at 50% of the peak intensity, for the green curve, d₅₀, ranges from approximately -3.0 inches to approximately 3.0 inches for a d₅₀ pattern diameter of approximately 6.0 inches. The pattern at 10% of the peak intensity, d₁₀, ranges from -5 inches to 5 inches for a d₁₀ pattern diameter of 10 inches. The IEC requirement is d₅₀ ≥ 0.5 x d₁₀. Substituting the pattern size values yields: 6.0 ≥ 0.5 x 10. As can be seen, the minimum IEC requirement is satisfied. 6.0 inches is greater then one half of 10 inches.

Paragraph 50.102.1.3 of IEC 60601-2-41 describes a surgical lighthead test to simulate cavity penetration. The test attempts to simulate the ability of a surgical lighthead to deliver light to the bottom of a surgical wound. The test is conducted by placing a tube, approximately 3 inches high and 2 inches in diameter, over the top of the photometer as shown in Figure 12. The photometer is placed approximately one meter or 39.4 inches from the bottom of the lens. Peak Intensity is measured with and without the tube in place. Cavity penetration is then given by the following ratio as tested according to IEC:

Figure 12: IEC Cavity Penetration Test

Paragraph 50.102.1.3 of IEC 60601-2-41 also describes a surgical lighthead test to simulate shadow control or shadow dilution. The test attempts to simulate the ability of a surgical lighthead to minimize the effects of shadowing when the light emitted from the surgical lighthead is blocked by members of the surgical team. The test is conducted with one or two masks blocking the light from the lighthead and with and without the tube used for cavity penetration. Test setup for one mask is illustrated in Figure 13. Shadow control or shadow dilution for one mask is then given by the following ratios as tested according to IEC:

Test setup for two masks is illustrated in Figure 14. Shadow control or shadow dilution for two masks is then given by the following ratios as tested according to IEC:

Figure 13: IEC Shadow Control or Shadow Dilution Test for One Mask
(Dimensions are Inches)

Figure 14: IEC Shadow Control or Shadow Dilution Test for Two Masks
(Dimensions are Inches)

Paragraph 50.102.1.3 (i) of IEC 60601-2-41 describes a surgical lighthead test to determine depth of field or depth of illumination. The test is conducted by measuring the peak intensity at one meter or 39.4 inches with the photometer. The photometer is then moved toward or away from the lighthead until 20% of the peak intensity is measured.

Figure 15 illustrates the IEC Depth of Field or Depth of Illumination Test. L₁ is the position closest to the lighthead where the 20% peak intensity value is obtained. L₂ is the position farthest from the lighthead where the 20% peak intensity value is obtained. Depth of Field or Depth of Illumination is the sum of L₁ + L₂.

Figure 15: IEC Depth of Field or Depth of Illumination Test

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