5B-i 

APPENDIX 5B. MODIFIED SPECTRUM INCANDESCENT REFLECTOR LAMPS 

TABLE OF CONTENTS 

5B.1 INTRODUCTION .........................................................................................................5B-1

5B.2 BACKGROUND ...........................................................................................................5B-1

5B.2.1 Spectral Power Distribution and Commonly Used Color Properties...........5B-1

5B.2.2 Methods of Modifying Spectral Power Distributions ..................................5B-4

5B.3 APPLICATION OF “MODIFIED SPECTRUM” DEFINITION TO IRLS .................5B-4

5B.3.1 Color Requirements .....................................................................................5B-5

5B.3.2 Black-Body Locus Requirements ................................................................5B-5

5B.3.3 MacAdam-Step Requirements .....................................................................5B-6

5B.4 METHODOLOGY FOR STANDARD AND MODIFIED SPECTRUM 

COMPARISON .......................................................................................................5B-7

5B.5 SELECTION OF SCALING FACTOR.......................................................................5B-12

REFERENCES ........................................................................................................................5B-13

LIST OF FIGURES 

Figure 5B.2.1 Spectral Power Distribution of Two Tested IRLs ..............................................5B-2

Figure 5B.2.2 The C.I.E. 1931 Chromaticity Diagram ..............................................................5B-3

Figure 5B.3.1 Combined Black-Body Locus and Color Requirements of the “Modified 

Spectrum” Definition ...................................................................................5B-6

Figure 5B.3.2 Example of IRLs That Meet the Definition of “Modified Spectrum;” 

Modified Spectrum IRLs Color Points Lie to the Lower Left of the 

Red Circle ....................................................................................................5B-7

Figure 5B.4.1 Transmittance of a Specific Lamp Cover Glass Featuring Neodymium, 

Relative to the Standard Spectrum Cover Glass Transmittance ..................5B-9

Figure 5B.4.2 Relationship Between Light Output Reduction and MacAdam-Step Shift 

For Lamps Tested ......................................................................................5B-11

LIST OF TABLES 

Table 5B.4.1 MacAdam-Step Shifts and Light Output Reductions for Lamps Tested ...........5B-10
5B-1 

APPENDIX 5B. MODIFIED SPECTRUM INCANDESCENT REFLECTOR LAMPS 

5B.1 INTRODUCTION 

A modified spectrum incandescent lamp is an incandescent lamp with special coatings or 

other features that modify the wavelengths of light that exit the lamp, changing the perceived 

color of the lamp’s light. These lamps are noticeably different in color than unmodified spectrum 

(or standard spectrum) incandescent lamps. Some incandescent reflector lamps (IRLs) are sold 

with features that modify their spectra, and the U.S. Department of Energy (DOE) believes that 

such IRLs provide a unique utility to users. For example, a number of these lamps are marketed 

as being able to reproduce the spectrum of natural daylight. The features that modify the spectra 

of incandescent lamps usually reduce the efficacies of the lamps. Thus, DOE created a modified 

spectrum IRL product class and established candidate standard levels (CSLs) for these lamps by 

scaling from CSLs established for the standard spectrum IRL product classes. The same 

approach was taken in setting standards for modified spectrum IRLs in the first cycle of 

amendments to IRL standards published in a final rule in July 2009 (hereafter the “2009 Lamps 

Rule”). 74 FR 34080 (July 14, 2009). 

 In the 2009 Lamps Rule, DOE determined the scaling factor that should be applied to 

efficacy requirements for standard spectrum IRLs to establish efficacy requirements for modified 

spectrum IRLs. Using the definition of “modified spectrum” specified in Code of Federal 

Regulations (CFR), 10 CFR 430.2, DOE determined what lamps would be included in the 

modified spectrum IRL product class. DOE concluded in that rulemaking that the definition of 

modified spectrum was created for general service incandescent lamps (GSILs) because it

requires the testing of clear lamps as a reference for spectrum comparison, and IRLs are not clear 

lamps due to their reflective coating. Therefore, DOE adapted the definition of “modified 

spectrum” for IRLs and developed the factor used to set the efficacy requirements for the 

modified spectrum IRL product class. In particular, DOE tested IRLs marketed as featuring 

spectral modification (such as lamps marketed as “daylight” lamps) and used similar standard 

spectrum IRLs as references for spectrum comparison. The following sections discuss the 

methodology used in the 2009 Lamps Rule to develop the scaling factor for the modified 

spectrum IRL product class. 

5B.2 BACKGROUND 

 Objects absorb, reflect, and radiate a form of energy known as electromagnetic radiation, 

which can be visualized as waves of energy that travel through open space. Different 

wavelengths of electromagnetic radiation have different common names; for example, radio 

waves have wavelengths ranging from millimeters to hundreds of thousands of kilometers, and 

visible light has wavelengths roughly between 380 and 750 nanometers (nm). Various 

wavelengths of visible light are perceived as various colors by humans. 

5B.2.1 Spectral Power Distribution and Commonly Used Color Properties 

 Some portion of the electromagnetic radiation emitted by lamps exists in the spectrum of 

visible light. Most lamps emit multiple wavelengths of light simultaneously, and the radiant 
5B-2 

power (energy radiated per unit time) of each wavelength can be plotted as a spectral power 

distribution (SPD).1Example SPDs are shown in Figure 5B.2.1 for a specific standard spectrum 

IRL and a specific modified spectrum IRL tested for the 2009 Lamps Rule. 

350 400 450 500 550 600 650 700 750 800

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

Wavelength (nm)

Spectral Power (W/nm)

Modified spectrum

Standard spectrum

Figure 5B.2.1 Spectral Power Distribution of Two Tested IRLs 

 The measurement of a lamp’s SPD allows the characterization of the lamp’s perceived 

light color. Three color properties in particular are commonly computed for lamps based on the 

SPD. One is the chromaticity, which refers to the color of the lamp’s light as indicated by a 

mapping onto a chromaticity diagram such as the Commission Internationale de l’Eclairage 

(C.I.E.) 1931 chromaticity diagramashown in Figure 5B.2.2.

2

Subtle differences in color can be 

quantified using MacAdam steps, named after David MacAdam, who studied color perception in 

the 1940s.3 A 3-step MacAdam ellipse is an ellipse that encircles a region of points on the 

chromaticity diagram that appear to be indistinguishable to most humans (in other words, they 

appear to be the same color). Points further than a distance of three MacAdam steps from any 

reference color point are generally perceived by most humans as being different in color than the 

reference point. 

 

a

 This particular figure is the chromaticity diagram for the CIE 1931 color space, created by the Commission 

Internationale de l’Eclairage (International Commission on Illumination, or CIE) in 1931. The colors on the outer 

curved boundary are composed of single wavelengths of electromagnetic radiation, with wavelengths indicated by 

the numbers. All colors inside the space are composed of a distribution of wavelengths of various powers. 
5B-3 

Figure 5B.2.2 The C.I.E. 1931 Chromaticity Diagram 

The curved line in the center of Figure 5B.2.2 is known as the black-body locus and 

represents the perceived light color emitted by a black body heated to the indicated temperature 

(in kelvin). In the field of physics, a black body is a theoretical object that is considered to be an

ideal emitter of electromagnetic radiation. It absorbs all electromagnetic radiation falling onto it 

(and thus reflects no radiation), and wavelengths of the radiation emitted by the body are related 

only to its temperature. Black bodies are so named because at room temperature they would emit 

mostly infrared radiation, which has a lower frequency and longer wavelength than visible light. 

Because they would also reflect no radiation, they would appear black. As the temperature of a 

black body increases, its emitted radiation increases in frequency while shortening in wavelength 

and begins to be perceived as visible light. Any object hot enough to emit visible light is said to 
5B-4 

incandesce, and the phenomenon of visible-light emission due to high temperature is known as 

incandescence. 

 A lamp’s correlated color temperature (CCT) is another commonly computed color 

property and is based on the behavior of black bodies. The CCT of a lamp is the temperature of a 

black body that produces roughly the same SPD of light (and thus roughly the same perceived 

color). Incandescent lamps produce light when electric current is sent through the filament inside 

the lamp, causing the filament to heat up and incandesce. The SPD generated by incandescent 

lamps is similar to the SPD generated by a black body; thus, the temperature of the heated 

filament is near the CCT of the lamp. Many IRLs have a CCT between 2,700 K and 3,000 K. 

The color rendering index (CRI) of a lamp is also commonly computed and refers to how 

closely the perceived color of an object illuminated by that lamp matches the perceived color of 

the same object illuminated by daylight or black-body radiation. The CRI ranges from 0 to 100; 

lamps with high CRIs illuminate objects such that humans perceive colors similar to what they 

would perceive if those objects were illuminated by daylight, or by a black body radiator. Most 

incandescent lamps without modified spectrums have a CRI near 100. 

5B.2.2 Methods of Modifying Spectral Power Distributions 

 The SPDs that can be generated by incandescence are such that incandescing objects can 

only produce a narrow subset of all of the colors that the human eye can perceive. To produce 

colors that are distant from the black-body locus, the SPD of an incandescent object’s radiation 

must be modified in some way. Various methods are used to modify the SPD of IRLs. Some 

methods utilize specific chemicals, such as neodymium, that are introduced into the cover glass 

of the lamp. Other methods rely on layers of material that are sandwiched into the cover glass or 

coatings that are applied to the outside or inside faces of the glass. The compounds, layers, or 

coatings used to modify the SPD operate by reducing the power of specific wavelengths of light; 

thus, they act as selective filters. Because the total light output of the IRL is reduced when 

selective filtering is applied, IRLs with modified spectrums are less efficacious than standard 

spectrum IRLs of the same filament type, bulb shape, rated voltage, and rated wattage. 

5B.3 APPLICATION OF “MODIFIED SPECTRUM” DEFINITION TO IRLS 

 The definition of “modified spectrum” as specified in the CFR is as follows: 

“Modified spectrum means, with respect to an incandescent lamp, an incandescent lamp that— 

 (1) Is not a colored incandescent lamp; and 

 (2) When operated at the rated voltage and wattage of the incandescent lamp— 

(A) Has a color point with (x,y) chromaticity coordinates on the C.I.E. 1931 

chromaticity diagram, figure 2, page 3 of IESNA LM-16 (incorporated by 

reference; see § 430.3) that lies below the black-body locus; and 

(B) Has a color point with (x,y) chromaticity coordinates on the C.I.E. 1931 

chromaticity diagram, figure 2, page 3 of IESNA LM-16 (incorporated by 

reference; see § 430.3) that lies at least 4 MacAdam steps, as referenced in 

IESNA LM-16, distant from the color point of a clear lamp with the same 

filament and bulb shape, operated at the same rated voltage and wattage.” 
5B-5 

10 CFR 430.2 

 The following sections will address each component of this definition in turn as it relates 

to modified spectrum IRLs. 

5B.3.1 Color Requirements 

 As noted in the definition of “modified spectrum” a modified spectrum lamp cannot be a 

“colored incandescent lamp.” The statutory definition of “colored incandescent lamp” is as 

follows:

“Colored incandescent lamp means an incandescent lamp designated and marketed as a colored 

lamp that has— 

(1) A color rendering index of less than 50, as determined according to the test method 

given in C.I.E. 13.3 (incorporated by reference; see § 430.3); or 

(ii) A correlated color temperature of less than 2,500K, or greater than 4,600K, where 

correlated temperature is computed according to the “Computation of Correlated Color 

Temperature and Distribution Temperature,” Journal of the Optical Society of America, 

(incorporated by reference; see § 430.3).”

10 CFR 430.2 

 Most IRLs with unmodified spectrums have a CRI near 100 and a CCT between 2,700 K 

and 3,000 K, and therefore would not be considered colored incandescent lamps. Colored 

incandescent lamps are lamps that have been modified, using coatings or other methods, so that 

they produce light that is perceived as blue, red, green, or other colors. These lamps are used for 

aesthetic or artistic effects and thus are not meant to reproduce object colors accurately. 

Although colored incandescent lamps are modified, they are excluded from the definition of 

modified spectrum. 

5B.3.2 Black-Body Locus Requirements 

 The “modified spectrum” definition also requires that a modified spectrum incandescent 

lamp produce a perceived color with a “color point ... that lies below the black-body locus” on 

the C.I.E. 1931 chromaticity diagram when the lamp is operated at its rated voltage and wattage. 

10 CFR 430.2 The chromaticity diagram and black-body locus are shown in Figure 5B.3.1, along 

with red straight lines indicating color requirements. The combined color and black-body locus 

requirements of the “modified spectrum” definition exclude all lamps with color points outside 

of the dark grey area in the figure. 
5B-6 

1931 CIE Chromaticity Diagram

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

x

y

Figure 5B.3.1 Combined Black-Body Locus and Color Requirements of the “Modified 

Spectrum” Definition 

5B.3.3 MacAdam-Step Requirements 

Finally, the “modified spectrum” definition requires that a modified spectrum 

incandescent lamp have a color point that “lies at least 4 MacAdam steps ... distant from the 

color point of a clear lamp with the same filament and bulb shape, operated at the same rated 

voltage and wattage.” 10 CFR 430.2 Requiring the color point to be “at least 4 MacAdam steps 

... distant from the color point of a clear lamp” ensures that modified spectrum lamps will be 

noticeably different in color than similar clear lamps. 

Figure 5B.3.2 shows a magnification of the C.I.E. 1931 chromaticity diagram with an 

example of three IRL samples that meet the definition of “modified spectrum.” The color points 

of the three standard spectrum and three modified spectrum IRL samples in this pair are shown 

as blue dots in the figure. The modified spectrum IRLs have color points located below the 

black-body locus, shown by the black line. In addition, they meet the color requirements 

bounded by the red straight line at the right (the left bound is located outside of the area shown 

by the figure). Finally, their color points meet the MacAdam-step requirement established by the 

three corresponding standard spectrum lamps located in the center of the red ellipse; the red 

ellipse represents a distance of four MacAdam steps from the average color point of the standard 

spectrum IRLs. 
5B-7 

Figure 5B.3.2 Example of IRLs That Meet the Definition of “Modified Spectrum;” 

Modified Spectrum IRLs Color Points Lie to the Lower Left of the Red Circle 

Applying the definition of “modified spectrum” requires comparing the color point of the 

IRL in question to the color point of a clear lamp. IRLs, however, are not clear lamps because 

they have reflectors; IRLs also commonly have frosted or textured faces. In order to apply the 

definition of “modified spectrum,” DOE developed a normalization method to model a clear 

lamp in the 2009 Lamps Rule. This method is discussed as part of section 5B.4. 

5B.4 METHODOLOGY FOR STANDARD AND MODIFIED SPECTRUM 

COMPARISON 

 To explore the efficacy differences between standard spectrum and modified spectrum 

IRLs, DOE paired commercially available standard spectrum IRLs with available IRLs in the 

same product line that were marketed as featuring spectral modification (for example, IRLs 

marketed as “daylight” or “full-spectrum” lamps) in manufacturers’ catalogs or marketing 

literature. Standard spectrum lamp models were paired with the lamps featuring spectral 

modification such that the lamps in each pair had the same bulb shape, the same rated life, the 

same rated voltage and wattage, and the same class of filament shape. All lamps had a rated 

voltage of 120 volts.

 In total, DOE tested seven pairs of parabolic aluminized reflector (PAR) lamps from two 

major manufacturers. Six pairs consisted of halogen lamps, and one pair consisted of halogen 

infrared (HIR) lamps. Each pair contained three samples of an IRL model marketed as featuring 
5B-8 

spectral modification along with three samples of a corresponding standard spectrum IRL model 

in the same product line. Halogen and HIR lamps were selected because these design options are 

required to meet the standards adopted by the 2009 Lamps Rule. DOE also tested one lamp pair 

featuring a smaller diameter (PAR16) than IRLs covered by this rulemaking, in order to 

supplement its analysis of larger-diameter lamps. 

 DOE measured and recorded the SPDs of the standard spectrum IRL samples in each 

pair, in accordance with DOE test procedures as specified in 10 CFR Appendix R to Subpart B 

of Part 430. Using the same guidelines, DOE also measured and recorded the SPDs of the 

samples of lamps marketed as featuring spectral modification. DOE then utilized the SPDs to 

compute the MacAdam-step shift and corresponding lumen output reductions between the 

standard spectrum and spectrally modified lamps across the lamp samples in every lamp pair. 

Because the MacAdam-step shift specified in DOE’s definition of modified spectrum 

lamp is relative to a clear lamp, DOE had to account for the fact that standard spectrum IRLs are 

not clear lamps due to the impact of their cover glass and their reflectors. DOE thus developed a 

methodology to isolate the effects of spectrum modification in order to determine the relative 

reduction in lumen output and the relative change in color that would be observed if a spectrally 

modified IRL were compared to a clear lamp. The method also assumes that spectrum 

modification is achieved through the usage of neodymium in the cover glass of a modified 

spectrum lamp. DOE used a model that simulates the SPD of a halogen capsule without any 

cover glass, termed HSPD in the equation below. This SPD is then used to simulate a clear lamp. 

The methodology utilizes the equation shown below in Eq. 5B.1: 

NW C G SPD

780

380

T

c

5

9

1

SPD N C G SPD M***

1e

*c 10 M**H *SA *

2































































































Eq. 5B.1

Where: 

2

1 2c  hc

 Wm

2 

k

hc

c2 

 mK 

 c = 2.99792458 ×108 m/s (velocity of light) 

 h = 6.626176 ×10-34 Js (Planck constant) 

 k = 1.380662 ×10-23 J/K (Boltzmann constant) 

and: 

 HSPD: the simulated spectral power distribution of a halogen or HIR capsule 

 MSPD: the measured spectral power distribution of a spectrally modified lamp 

: wavelength of light emitted from the lamp filament (in meters) 

 T: temperature of the filament (in kelvin) 
5B-9 

SA: surface area of the filament (in square meters), determined by selecting a black-body 

radiator with the same radiant power as the measured standard spectrum IRL 

α: correction term for the filament surface area 

εW: emissivity of tungsten, which models a tungsten filament 

τN: transmittance of neodymium in a spectrally modified cover glass 

τCG: transmittance of a clear cover glass 

β: combined effect of neodymium concentration and thickness of neodymium in a 

spectrally modified cover glass 

Transmittance is the fraction of light (at a specific wavelength) hitting an object that 

travels through the object and can be measured on the other side. To determine the transmittance, 

DOE measured the transmittance of a standard spectrum cover glass, τCG, between the 

wavelengths of 380 nm and 780 nm. DOE also measured τN using a spectrally modified lamp 

cover glass. The cover glass transmittance for a specific lamp featuring spectral modification 

tested by DOE is shown relative to the corresponding standard spectrum lamp transmittance in 

Figure 5B.4.1. Sharp dips in the plot clearly indicate which light wavelengths are absorbed by 

the combination of neodymium and cover glass. 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

380 430 480 530 580 630 680

Spectral Transmittance

Wavelength (nm)

Figure 5B.4.1 Transmittance of a Specific Lamp Cover Glass Featuring Neodymium, 

Relative to the Standard Spectrum Cover Glass Transmittance 

DOE utilized an unconstrained nonlinear optimization program to compute α, β, and T 

and thus model the SPD of the capsule inside of a spectrally modified lamp, HSPD. DOE then 

used HSPD and the transmittance of a cover glass without any spectrum-modifying features in Eq. 

5B.2 in order to obtain ISPD, the SPD of a clear lamp: 
5B-10

SPD CG SPD  I*H

Eq. 5B.2

The light output reduction is obtained by dividing MSPD by ISPD. In addition, both ISPD and MSPD

were used to calculate color points. Then, the color separation from the ISPD color point to the 

MSPD color point was determined in terms of MacAdam steps. 

 DOE determined the relationship between MacAdam-step shift and light output 

reduction, as shown in Table 5B.4.1 and Figure 5B.4.2, for every individual lamp combination 

tested (21 combinations of standard spectrum and spectrally modified lamps, derived from the 

seven lamp pairs that each contained three standard spectrum and three spectrally modified lamp 

samples).

Table 5B.4.1 MacAdam-Step Shifts and Light Output Reductions for Lamps Tested 

Lamp ID

alpha beta T

MacAdamStep Shift

Light 

Output 

Reduction

Standard 

Spectrum

Spectrally 

Modified

105352 105349 1.249 1.076 2,746.221 5.4 19.2%

105353 105350 1.239 1.146 2,746.022 5.7 20.1%

105354 105351 1.290 1.116 2,741.640 5.6 19.7%

105374 105371 1.323 1.086 2,715.983 5.5 19.3%

105375 105372 1.344 1.096 2,707.611 5.5 19.5%

105376 105373 1.273 1.029 2,726.019 5.2 18.6%

106705 106702 1.275 0.981 2,845.517 5.0 17.9%

106706 106703 1.296 1.066 2,838.923 5.4 19.0%

106707 106704 1.271 1.055 2,857.426 5.4 18.9%

106751 106748 1.498 0.473 2,699.566 2.4 10.0%

106752 106749 1.488 0.374 2,678.070 1.8 8.2%

106753 106750 1.803 0.395 2,653.760 1.9 8.6%

105355 105276 1.337 0.425 2,739.313 2.1 9.1%

105356 105277 1.411 0.500 2,750.256 2.5 10.5%

105357 105278 1.305 0.430 2,751.663 2.1 9.2%

105358 105269 1.315 0.456 2,818.707 2.3 9.7%

105359 105270 1.272 0.448 2,818.681 2.3 9.5%

105360 105271 1.256 0.473 2,821.212 2.4 10.0%

105363 105287 1.208 0.547 2,760.137 2.8 11.3%

105364 105288 1.337 0.541 2,784.910 2.7 11.2%

105377 105289 1.325 0.495 2,770.849 2.5 10.4%
5B-11 y = 44.091x1.2694

R2 = 0.9997

0

1

2

3

4

5

6

7

0% 5% 10% 15% 20%

MacAdam Steps

Light Output Reduction 

Figure 5B.4.2 Relationship Between Light Output Reduction and MacAdam-Step Shift For 

Lamps Tested 
5B-12

5B.5 SELECTION OF SCALING FACTOR 

 The results of DOE’s analysis of commercially available lamps for the 2009 Lamps Rule 

indicated that a 4-MacAdam-step shift between a standard spectrum IRL and a lamp featuring 

spectral modifications corresponded with an approximate efficacy reduction of 15 percent, which 

was the scaling factor used in the 2009 Lamps Rule. For this rulemaking, DOE is maintaining 15 

percent as the efficacy reduction factor required of IRLs that meet the “modified spectrum” 

definition. A comparison of the baseline and candidate standard level put forth in this rulemaking 

for standard spectrum and modified spectrum IRLs with this scaling factor applied is shown in 

Figure 5B.5.1. 

Figure 5B.5.1 Comparison of Standard Spectrum and Modified Spectrum for Candidate 

Standard Level 1 
5B-13

REFERENCES 

1 Rea, Mark S., ed. The IESNA Lighting Handbook: Reference & Application, 9th Edition. 2000.

The Illuminating Engineering Society of America: New York, New York. 

2

 International Energy Agency. Light’s Labour’s Lost: Policies for Energy-efficient Lighting.

OECD/IEA, 2006. 

3 D. L. MacAdam, "Visual Sensitivities to Color Differences in Daylight," J. Opt. Soc. Am. 32, 

247-273 (1942).