Lighting Systems for Fruit Sorting 



Daniel Guyer, Roger Brool(, and Edwin Timm 



Agricultural Engineering Department, Michigan State University 



This article is modified from one that appeared in the Washington State University Tree Fruit 

 Postharvest Journal, Vol. 5, No. 1, which was modified from Michigan State University -- Cooperative 

 Extension Service Agricultural Information Series, AEIS 618, January, 1994. 



Fruits and vegetables are inspected prior to most 

 processing or packing operations. While some sorting 

 is accomplished with optical or electronic technology, 

 much sorting is done by manual visual inspection. Each 

 woricer must look at a few hundred items each minute 

 and accurately discard those that are unacceptable. 

 Good lighting conditions are required to perform this 

 task. 



Sorting table lighting may not currently match the 

 specific task for which it is intended. Specific guide- 

 lines for lighting system design in fruit and vegetable 

 sorting and packinglines in the U.S. do not exist. 

 Manufacturers of packingline equipment have left 

 lighting decisions up to the individual operation. 



Sorting table lighting must have both adequate 

 intensity and color quality to enhance or reveal defects 

 rather than to obscure or mask them. Improper lighting 

 design promotes woilcer fatigue and eye strain, result- 

 ing in poor sorting efficiency. Studies of several 

 operations involving inspection of a range of commodi- 

 ties have shown that many lighting systems are not 

 adequate for the required task. These studies suggested 

 that improved sorting results could be expected if 

 relatively inexpensive changes in illumination sources, 

 illumination intensities, and background colors were 

 adopted in sorting areas. 



Principles of Lighting and Color 



Two common uses of lighting are (a) general area 

 lighting, and (b) task lighting. General area lighting's 

 purpose is to illuminate a room or building for general 

 activity. This type of lighting is usually mounted in the 

 ceiling or well above the floor area. Task lighting is 

 much more specific and is concentrated in an area to 

 enhance the ability to perform a task. Task lighting is 

 the primary concern of this article which focuses on the 

 task of manually sorting fruits and vegetables. 



Three major components interact in the process of 

 visualizing a "color": 



1 . li ght energ y from a lamp or light fixture; 



2. color reflectance potential of a fruit, caUed 

 spectral reflectance : and 



3. sensitivity of the eye to color, called receptor 

 sensitivitv. 



For example, to "see" the color red there must exist a 

 light source containing red color light, a surface which 

 can reflect the red light and a receptor sensitive to 

 reflected red light. 



Light Energy 



Light energy, or a source of light, is required to 

 produce the actual visible color light which the eye can 

 detect The natural light source is the sun which 

 produces all visible colors in addition to energy outside 

 the visible spectrum (ultraviolet, infrared, etc.). 

 Colorsproduced by artificial light are influenced by 

 tube coatings, such as phosphor in fluorescent tubes, 

 gases, or other components contained in filament bulbs. 

 Artificial light sources are rated by: 



1. Color temperature; black body temperature 

 generation; 



2. CRI: Color Rendering Index; and 



3. CPI: Color Preference Index. 



These ratings are explained briefly in the footnotes of 

 Table 1. 



Of the three major components in visualizing color, 

 light energy is the one most easily controlled. The 

 important factor relating to artificial light is the spectral 

 irradiance curve for a given light source. A spectral 

 irradiance curve is a measured representation of a given 

 light source showing the amount of specific light en- 

 ergy or color contained in the source over the spectrum 

 of colors. Spectral irradiance curves are generally 

 available from lamp manufacturers. The spectral irra- 



Fruh Notes, Fall, 1994 



