Lighting Systems for Fruit Sorting
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 worker 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 guidelines 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 obscure or mask them. Improper lighting design promotes worker fatigue and eye strain, resulting in poor sorting efficiency. Studies of several operations involving inspection of a range of commodities 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
Three major components interact in the process of visualizing a "color":
- Light energy from a lamp or light fixture
- Color reflectance potential of a fruit, called spectral reflectance
- Sensitivity of the eye to color, called receptor sensitivity
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.
Artificial light sources are rated by:
- Color temperature; black body temperature generation
- CRI: Color Rendering Index
- CPI: Color Preference Index
These ratings are briefly explained in the footnotes of Table 1.
Table 1. Artificial lighting characteristics and visual effects on common produce colors, 1992.*
|Visual effect on specified color|
|Warm White Dlx||2||2.1||3000||79||90||68||E||E||D||E||D||E||E|
|Cool White Dlx||2||3.2||4200||89||94||70||E||E||D||D||E||E||D|
1=General Electric; 2=Phillips; 3=Duro Test.|
Rel cost: Relative bulb cost ration to Cool White.
Color temp: Lamp appearance in degrees Kelvin.
CRI: Color Rendering Index=effect the light source has on appearance of colored objects; 100=perfect appearance
CPI: Color Preference Index=how well people recognize colors in that light; 100=perfect recognition.
Rel light: Relative initial lumen/watt output as a percentage of Cool White.
Ma=Maroon; Re=Red; Gr=Green; Br=Brown; Bl=Blue; Pu=Purple; Ye=Yellow
Visual effect of tube on specified color: B=brownish cast; D=darker; E=enhanced; W=whitish cast. Cool White effects are relative to midday diffuse outdoor light; other tubes are relative to Cool White.
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 energy or color contained in the source over the spectrum of colors. Spectral irradiance curves are generally available from lamp manufacturers. The spectral irradiance for a light source can be altered with various types of "filters" covering the lamp. These include undesirable coatings of dust and dirt.
Many defects that need to be detected on fruits and vegetables are of brown or grayish color.
One might assume, therefore, that simply finding the light source with the most energy in the color regions making up the brown color would be ideal for all applications. The objective in selecting the best light source for a given task, however, is to light a commodity with a source that will accentuate the color difference between the sound tissue and the defects. For example, if we wish to find brown discoloration on red cherries, then we want to use an inspection light of a color that will accentuate brown against the normal red color of the cherry. The key is to find a color of inspection lighting that will make the defects show up the most, i.e., to make the commodity look its worst.
Performance of Commercially Available Light Sources
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, Daylight, Natural, Optima 32, Optima 50, C-50, and C-75. Consequently, these lights should not 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 1994 or 1995 because they do not meet proposed efficiency levels. Fluorescent tubes of 8-foot lengths of all types are also 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 appropriate choice for most sorting operations when both sorting performance and lighting cost are considered (Figure 1a). Note how the spectral irradiance curve of SP-30 closely matches the perceived cherry color.
Except for metal halide, the high intensity discharge (HID) lights were undesirable for produce sorting 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 recognition and enhanced ability to see brown-colored defects on dark-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
- Background color of sorting surface (belt). Reflected light energy from the sorting surface should not be greater than that from the produce. (Note: some food handling systems are required to have white belts.) Use belts which are black or dark gray, but not glossy finish.
- Surrounding colors. Surfaces near sorting areas and the clothing of inspection personnel should not be bright or highly reflective and should not cause glare.
- Placement of fixtures. Placement should be such that the light source will not be directly in the sorters' eyes, i.e., unshielded, or too low so as to obstruct the sorters' view of the sorting surface. The fixture must also be placed at such a height as to provide the proper level of light at the sorting surface. This will depend on the amount and type of light used and the considerations mentioned above. For an SP-30 light, this height will be about 32 inches above the sorting surface, as shown in Figure 2.
- Type of lighting. Light type should be appropriate for the sorting task and the colors involved. Area lighting should also be considered as it can have negative impacts on the color evaluation and on eyestrain.
- Screen, block, or direct all task light sources ao that they cannot glare in the workers' eyes.
- Use SP-30 (or equivalent) illumination at the sorting area of most fresh produce packinglines.
- Adjust lamp power levels (number of tubes) and fixture height so that lightly colored produce receives approximately 250 foot-candles of illumination and darkly colored produce, approximately 500 foot-candles.
- Minimize the influence of natural, stray, and general area lighting in the sorting area.
- Select similar dark colors for equipment parts and worker clothing in the sorting area so that bright areas cannot interfere with the workers' established vision conditions.
- Use a dark background color (black, gray, dark brown) on the conveyor surface carrying the produce so that reflected light energy from this surface is not graeter than that from the produce; avoid a glossy finish on the belt surface.
References for More Information
Affeldt, H. A. and P. W. Winner. 1991. Lighting practice and principles for manual citrus inspection. Paper No. 913549, ASAE, 2950 Niles Rd., St. Joseph, MI 49085.
Brown, G. K. 1991. Lighting for manual sorting of apples and sweet cherries. Paper No. 913553, ASAE, 2950 Niles Rd., St Joseph, MI 49085.
Brown, G. K., D. E. Marshall and E. J. Timm. 1993. Lighting for fruit and vegetable sorting. Paper No. 936069, ASAE, 2950 Niles Rd., St. Joseph, MI 49085.
Davies, J. and R. M. Perkins. 1991. Effect of illumination in grading dates. Paper No. 913547, ASAE, 2950 Niles Rd., St. Joseph, MI 49085.
Delwiche, M. J., J. F. Thompson and R. S. Johnson. 1991. Sorting table illumination on stone fruit packing lines in California Paper No. 913551, ASAE, 2950 Niles Rd., St. Joseph, MI 49085.
Hyde, G. A. 1991. Lighting environment for manual sorting of potatoes and onions. Paper No. 913548, ASAE, 2950 Niles Rd., St. Joseph, MI 49085.
Kantowitz, B. and R. Sorkin. 1983. Human factors, understanding people-system relationships. John Wiley & Sons, Inc., pp. 102.
Kupferman, E. M. 1991. Cherry sorting table lighting. Paper No. 913552, ASAE, 2950 Niles Rd., St. Joseph, MI 49085.
Daniel Guyer, Roger Brook, and Edward Timm
Department of Agricultural Engineering, Michigan State University
Tree Fruit Postharvest Journal 5(1):22-28