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Demystifying Daylight (Part 2)
By David Crowther

To continue our discussion on demystifying daylight, we have all ready looked at a couple of myths; Myth #1 - All daylight simulators are the same, and Myth #2 - As long as the light source is D65, it’s a good daylight simulator. Now, we can discuss the final two myths.

Myth #3 - CRI is the best way to determine the quality of a daylight simulator.
FALSE! Colour rendering index, primarily used for fluorescent lamps, represents a light source’s ability to render colour. It was developed by the lamp manufacturers themselves as a means to illustrate the colour rendering performance of their fluorescent lamps – theoretically, the higher the index, the better the colour rendering. However, a little known distinction in the way the index works can be misleading.

20438_4C.gifWhy some fluorescent light sources can have excellent colour rendering indexes, yet poorly render colour.
The colour rendering index is a relative index, meaning the lamp performance is relative to a reference light source, which is assigned the highest rating of 100. The reference light source for lamps below 5000 degrees Kelvin (most commercially available fluorescent light sources) is incandescent or tungsten. It’s a well-known fact that incandescent light has very little blue energy and an overabundance of yellow, orange and red energy – certainly not a good choice for accurate colour evaluation. Yet this light source has a CRI of 100. Caveat... the CRI index is not an absolute indicator of lamp performance. CRI tells you that lamp performance is only as good as the reference source, which may not be good at rendering colour at all.
For an absolute indication of how well a lamp renders colours, consult the CIE Assessment Method, Publication 53. The following is a summary.

CIE Assessment Method (Publication 53)  

  • Tests spectral quality of daylight
    simulators for visual appraisals and instrumental measurements.
  • Includes test methods for D50, D55, D65 and D75.
  • Uses 5 virtual metamer sets for visible and 3 for ultraviolet.
  • Quality grades are based on delta E or total colour difference of the metameric sets.
  • More accurate than CRI for evaluating the quality of daylight simulation. 

Quality Grade

Metamerism Index

 CIE Lab

 CIE Luv





 0.25 to 0.50

 0.32 to 0.65


 0.50 to 1.00

 0.65 to 1.30


 1.00 to 2.00

 1.30 to 2.60




The use of good quality daylight simulation is not only a good idea, but in many industries, it is an absolute requirement. This is further evidenced by the number of corporate, national and international visual colour standards requiring accurate daylight simulation including ASTM D1729, SAE J 361, BS950, ISO 3664 and AATCC

Procedure 9.
The following illustrates the daylight rating requirements of several industry standards:
• ASTM D1729 “Visual Evaluation of Colour Differences of Opaque Materials” (BC required)
• SAE J361 2000 “Visual Evaluation of
Interior and Exterior Trim” (rating of B and should be A)
• ISO 3664 - 2000 “Viewing conditions for Graphic Technologies and Photography” (C or better and should be B)
• BS 950 Part 1 (BD required)
• AATCC Evaluation Procedure 9 “Visual
Assessment of Colour Difference of
Textiles” (BC or Better)

FPL1_SYM 4C.gifMyth #4 Fluorescent technology is more advanced and works as well as the leading filtered tungsten halogen technology at a lower price.
FALSE! The upfront investment of filtered tungsten halogen technology is certainly more than fluorescent technology. However cost issues aside, the performance of filtered tungsten halogen technology is unparalleled by any fluorescent technology, including a lower cost yet highly accurate specialized seven-phosphor daylight simulation technology. One must consider the total cost of ownership plus consistent quality of colour rendering over the life of the product. Let’s take a look at the facts about fluorescent versus filtered tungsten halogen technology.

Why simply changing the lamps is not enough to ensure accurate colour rendering...

It’s fact that the colour rendering of any light booth is a function of the entire system – not just the lamps.
To ensure optimum system performance, certification is required by every quality system such as ISO, Six Sigma, etc. The certification process involves a comprehensive system check. All light sources are measured with a spectroradiometer for light levels, colour temperature, spectral power distribution and chromaticity coordinates. The system is thoroughly cleaned and operating voltages measured with a true RMS meter and set for proper operation. While all lighting systems are shipped with the correct colour temperature, many environmental factors can shift the spectral characteristics of the system including cleanliness of the walls and filters and performance of the ballast and power supply. Simply changing the lamps is analogous to the Department of Motor Vehicles sending you an inspection sticker for your car without your having the emissions, brakes, tires and lights checked.

Why lamp stability is not an issue with either technology...
Lamp stability, whether fluorescent or tungsten, is a function of the current supplied to the lamp. In fluorescent fixtures, the use of a ballast, commonly known as a constant wattage auto transformer, is required to start the lamps at voltages in excess of 800 volts. Once the arc is struck, the ballast maintains the proper operating voltages to the lamp. Every time the fluorescent lamp is turned on and off, the same process occurs. This is why it is better to leave fluorescent lamps on rather than turn them on and off.
ROEHREN_4C.gifTypical household tungsten or incandescent lamps are dependent on a constant current to maintain their colour temperature and light output. A reduction in the voltage to a tungsten lamp will decrease light output, colour temperature and increase lamp life. The opposite is also true. That’s why tungsten halogen colour viewing technology employs a highly engineered regulating power supply. Once calibrated at the factory, this system not only maintains constant power to the lamps, but also automatically corrects for high or low voltage based on the electrical load in a factory or office environment. These “smart” systems literally check for proper line voltage on start-up and notify operators if the voltage supply falls below acceptable limits at any time during operation. Thus, voltage fluctuations are eliminated from affecting colour judgment over the life of the system.

Why lamp life can be misleading...
The objective of colour viewing is to have a reliable consistent daylight source. Longer lamp life does not mean more hours of reliable consistent colour rendering. On the contrary, most fluorescent lamps have a much longer lamp life than the 400 hours of tungsten halogen. However, approximately 75% of the lamp life is not usable for colour viewing. Here’s why...
Studies show that fluorescent lamps experience as much as a 30% reduction in light output and colour shifts as high as 400 degrees Kelvin over the life of the lamp. In addition, the phosphors within the lamp degrade over time. This degradation reduces the lamps efficiency and changes the spectrum produced. The first change affects the red and orange regions of the spectrum. Red content is reduced, light output is reduced and the ultraviolet content is increased. The change occurs gradually over time and is not apparent to the naked eye. However, it can unknowingly have a profound affect on colour judgment. It is recommended that fluorescent lamps be changed every 3000 to 5000 hours to meet the light level, colour temperature and spectral output for accurate colour rendering.

There’s a lot more to daylight than meets the eye. Critical colour decisions, on which millions in profit and loss are at stake, require careful consideration and a clear understanding of the advantages and limitations of simulated daylight technology.
That’s why numerous corporations and industries worldwide specify colour viewing systems that meet and exceed specified industry standards (e.g. ISO 3664:2000) for reliable daylight simulation and to have the most confidence in accurate colour evaluation.

Note: This article draws in part from a technical paper written by Gretag Macbeth AG* - *an X-Rite company. This information and the accompanying images are reproduced with the permission of X-Rite.

David Crowther
Technical Manager - X-Rite products
Manager – Chromaticity Australia