Selling Apples

A Grocery is selling apples and is testing four different light fixtures for use in the Produce Dept. The apples below are identical, but ‘look’ different because the four light fixtures have different CRI values. Which one looks the best?
Why do these same four apples look so different?

Answer: The CRI ratings are different for each of the four lights.

WHAT IS COLOR RENDERING INDEX (CRI)?
To explain CRI we must first take a look at light. When you take a prism and let sun light go through it, you can see the full spectrum of the different colors that combine to form white light.
Optical testing equipment can create special color distribution graphs that show the relative amount of the different colors for a given light source. These graphs also reveal the ability of a light source to illuminate all, or, selected colors of the spectrum.
For example, below is a color distribution graph for daylight.
Notice the strong presence of ALL colors that combine to create daylight. With all of the colors of the full spectrum of light, one can see that daylight provides a high level of ‘accurate’ color illumination when an object is seen in daylight. These color distribution graphs show the relative ‘amount’ of the different colors that comprise the full visible spectrum for a given light source. These graphs also reveal the ability of a light source to illuminate all, or, selected colors. For example, below is a color distribution graph for fluorescent lighting.
The most obvious difference between these two color distribution graphs is the lower level of color in the fluorescent light when compared to daylight. You can see a few color ‘spikes’ in the fluorescent light, but only certain wavelengths (the spikes) are strongly represented.

These spikes indicate which parts of the color spectrum will be emphasized by the fluorescent light when in use to illuminate objects like the apples.

The ability of fluorescent lights to illuminate the color of apples with a CRI of 82 is not bad, but certainly much worse than daylight, which has a CRI closer to 100, depending on time of day. Notice the deep troughs in the graph for the fluorescent light where the curve almost reaches zero for certain colors.
 
CRI, or Color Rendering Index, is a numerical scale of 0 to 100 (Chart to right) used in lighting to indicate how a light source will make the color of an object appear to human eyes. The higher the number, the more ‘accurate’ and appealing the objects color appears. The highest attainable CRI is 100. Lamps with CRIs above 70 are typically used in office and living environments. A standard "cool white" fluorescent lamp will have a CRI near 62.
 
For certain applications such as illuminating apples, artwork or for shopping in retail clothing stores, a higher CRI can make all the difference.
Bill Nagengast, Lighting Engineer
Solas Ray Lighting
Holds over 20 patents in the lighting industry.
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TJ-46 10.12.18