Perceived Color 



Figure 1 demonstrates the combining of all compo- 

 nents (spectral irradiance, reflectance curves, and re- 

 ceptor sensitivity) which affect color perception. The 

 perceived color is termed the "total spectral energy 

 distribution" and is the product of the spectral irradi- 

 ance X spectralreflectance x human eye sensitivity or 

 response. The goal of the lighting design is to have 

 "peaks" in the distribution at the wavelengths or colors 

 of the commoditv and at the defect color, thus resulting 

 in a good perceivable contrast . Figure la shows the 

 peaks in the perceived color of a dark red cherry, and 

 Figure lb shows that these same peaks are strong in 

 light from an SP-30 light, so a good match exists. 



Performances of Commercially 

 Available Light Sources 



Theoretically the product of the spectral irradiance, 

 spectral reflectance, and eye sensitivity should provide 

 the information to design a proper lighting scheme. 

 USDA-ARS researchers at Michigan State University 

 evaluated several commercially available light sources. 

 They measured individual spectral irradiance, using 

 color chips to subjectively analyze and compare perfor- 

 mance in color perception tests. Table 1 siunmarizes 

 their findings and provides technical and relative cost 

 information. Results indicated that the "image" curves 

 resulting from the combination of spectral reflectance x 

 spectral power x eye sensitivity generally agreed with 

 the subjective/visual results for the test with the color 

 chips and real produce items. 



The ability to recognize differences between good 

 and defective areas on produce was lowest under Cool 

 White (CW) light, which was very similar to that for 

 CW Deluxe, Warm White, Warm White Deluxe, Day- 

 light, Natural, Optima 32, Optima 50, C-50, and C-75. 

 Consequently, these lights should qqi be used for task 

 lighting in fruit and vegetable inspection areas. U.S. 

 federal energy standards may eliminate CW and similar 

 type fluorescent lamps by 1995 because they do not 

 meet proposed efficiency levels. Fluorescent tubes of 

 8-foot lengths of all types also are scheduled to be 

 removed from production. 



Visual color comparisons suggested that although 

 the SP-30 light had a low color rendering index (CRI), 

 it performed better than higher CRI fluorescent lights 

 for the visual sorting of most fruits and vegetables. The 



relative light output of the SP-30 lamp is among the 

 highest tested. Its relative cost is only 1.8 times that of 

 CW. These factors indicate that it should be an appro- 

 priate choice for most sorting operations when both 

 sorting perfoimance and lighting cost are considered 

 (Figures la and lb). Note how the spectral irradiance 

 curve of SP-30 closely matches the perceived cherry 

 color. 



Except for metal halide, the high intensity dis- 

 charge (HID) lights were undesirable for produce sort- 

 ing as they severely darkened most colors. Tests will be 

 necessary using metal halide light to determine if 

 sorting performance is acceptable. Tungsten halogen 

 quartz (quartz) light also produced good color recogni- 

 tion and enhanced ability to see brown-colored defects 

 on daik-colored produce. Both metal halide and quartz 

 lighting will be more costly than SP-30 fluorescent 

 lighting. More specific discussion of the tests will not 

 be covered here but can be found in the cited reference. 



Requirements of Light Intensity 



The average illumination intensity needed on pro- 

 duce items foreffective visual sorting seems to be in the 

 range of 250 to 500 foot-candles, based on the reactions 

 of woricers 20 to 70 years old. The lower intensitv 

 seems adequate for li ght -colored (high reflectance) 

 produce, and the hi gher intensitv for dark-colored flow 

 reflectance) produce. The actual light intensity may 

 need to be adjusted, depending on the design consider- 

 ations discussed below. Insituations where kinds or 

 varieties of produce covering the entire color range 

 must be insp)ected on the same packing line, the low and 

 high intensity levels should be selectable by the sorting 

 woricers. This easily can be accomplished by using 

 four-tube fluorescent fixtures wired so that either the 

 two outside tubes or all four tubes can be turned on. The 

 amount of light falling upon a surfce can be measured 

 with commercially available light (foot-candle) meters. 



Design Considerations 



Several physical design characteristics will impact 

 on sorting efficiency and overall worker attitude and 

 performance. 



Background color of sorting s urface fl^elt). 

 Reflected light energy from the sorting surface should 

 not be greater than that from the produce. Use belts 



Fruit Notes, Fall, 1994 



