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Tables for calculating desired light flux densities for horticultural crops
Scientia Horticulturae, 1 (1973) 263-269 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
TABLES FOR CALCULATING DESIRED LIGHT FLUX DENSITIES FOR HORTICULTURAL CROPS
ANTON M. KOFRANEK* and MICHAEL ROBINSON
Department o f Ornamental Horticulture, The Hebrew University, Rehovot (Israel) *Permanent address: Department of Environmental Horticulture, University of California, Davis, Calif. 95616 (U.S.A.) (Received February 9th, 1973)
Light emissions in lux from single incandescent lamps ranging from 60 to 200 watts were arranged in tables according to the lamp height above the ground. The method of calculating the desired lamp spacing to achieve a desired light intensity is illustrated. A table showing the reduced light emission due to various voltage drops is given along with precautions to avoid them.
INTRODUCTION The photoperiodic requirements of many plants are well known, but very often it is not known how to space lamps to achieve the desired light flux densities to induce or prevent flowering. There is general agreement that incandescent light of relatively low light intensity may be ample to trigger a photoperiodic response. It is possible to calculate light flux densities by known formulae or to measure them empirically with sophisticated devices, but these tables are intended to aid those who wish to calculate a desired minimum light level without resorting to formulae or light meters. Therefore, these tables may be useful to growers, advisory service personnel or the horticultural scientist wishing to calculate the proper incandescent lamp spacing for purposes of supplying supplementary light to a certain crop. METHODS Light readings in lux were recorded with a Gossen light meter from single incandescent lamps suspended 1.0, 1.5, or 2.0 meters above the soil.
264 Readings were made with the light sensing device placed perpendicular (P) to the light source or with the device placed horizontally (H) on the soil surface. Light readings were made on a flat surface, in the dark location without any side reflective surfaces, at horizontal intervals o f 0.5 meter distant from the lamp until the readings were about 3 lux or less. Lamps o f clear glass o f various wattage ratings shown in the tables were used with or without a white dome reflector. The light measurements from each lamp source were assembled in the tables and arranged in relation to the lamp height above the ground, and the distance, starting from a point directly beneath the lamp (zero), measured away from zero in a horizontal direction. HOW TO USE THE TABLES First one must decide what minimun light flux density is necessary for the crop. F o r example, the chrysanthemum, a short-day plant, requires a minimum of 100 lux at 15 °C night temperature to inhibit premature flower bud initiation. Therefore, one must calculate the spacing o f the lamps to record 100 lux in the darkest location o f the glasshouse or field. The tables clearly illustrate that large lamps with or without reflectors, spaced high above the soil surface emit light further than smaller lamps. Note that there are t w o different lux values listed under each bulb size. The " P " value was recorded when the light sensing device was held perpendicular to the light source to give a m a x i m u m reading at a given distance. The " H " value was obtained with the device held flat on the ground at all points, and much o f the light was reflected o f f the glass surface of the sensing device at distant points. One can expect that the plant perceives light somewhere between these two values (H and P) depending on the position and orientation of the leaves, the amount of shading from other plants and the d i s t a n c e from the light source. If a dense canopy is to be lighted only at the plant tops, then the readings from the " H " column should be chosen. On the other hand, if the crop is sparsely planted and the light is able to penetrate at many angles, then use of the " P " column would be more appropriate. The decision as to which column should be used depends mainly on the growth habit o f the crop. If only a single bed is to be lighted in a greenhouse, one should choose small lamps placed close to the plants in order to confine the light to a narrow area. If one desires illumination o f the entire greenhouse or a large field, then it is wise to choose the 200-watt lamps suspended 2.0 meters above the growing surface for the most efficient light distribution. The light readings from t w o or more light sources u p o n one point are additive. For example, if t w o 200-watt lamps with reflectors are sus-
265 TABLES I, II, III The horizontal distance on the ground was measured from a point starting directly beneath the lamp (zero) and continuing in a straight line at 0.5 meter intervals. The "P" value was recorded with the sensing device always perpendicular to the light source (greatest possible reading) and the " H " value was recorded with the device laid horizontal on the soil surface at all distances.
TABLE Ia Light values in lux for various size lamps without reflectors placed 1.0 meter above the soil surface Horizontal distance from zero (meters)
zero 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Lamp size (watts): 60
75
100
150
200
P
H
P
H
P
H
P
H
P
H
50 40 23 14 8 6
50 30 15 7 4 2
60 46 32 18 12 8 6
60 42 22 10 5 3 2
92 46 42 25 17 12 8
92 58 30 13 6 4 2
180 120 76 48 27 19 14 11
180 100 52 24 9 7 3 2
200 180 120 60 38 27 19 15 11 9
200 140 76 30 18 11 7 4 3 2
TABLE Ib Light values in lux for various size lamps with a 30-cm-diameter white dome reflector placed 1.0 meter above the soil surface Horizontal distance from zero (meters)
zero 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Lamp size (watts): 60
75
100
150
200
P
H
P
H
P
H
P
H
P
H
160 90 50 26 14 4
160 70 40 14 6 2
280 120 58 26 10 4
280 100 40 13 5 1
340 200 93 44 22 5
340 180 62 20 10 2
500 310 160 76 36 11 5
560 250 100 42 18 6 1
570 400 210 100 54 38 24 11
570 330 140 50 22 14 5 2
266
•
TABLE IIa Light values in lux for various size lamps without reflectors placed 1.5 meters above the soil surfac.e Horizontal distance from zero (meters)
zero 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Lamp size (watts): 60
75
100
150
200
P
H
P
H
P
H
P
H
P
H
24 19 15 12 9 6
24 18 13 8 5 3
30 26 19 13 9 6 5
30 25 15 10 5 3 2
42 36 30 19 14 11 7
42 34 24 13 8 5 3
74 66 50 36 25 18 14 12 9
74 62 42 26 14 9 6 4 3
100 86 64 46 28 22 16 12 9 7
100 80 54 34 20 11 7 4 3 2
TABLE IIb Light values in lux for various size lamps with a 30-era-diameter white dome reflector placed 1.5 meters above the soil surface Horizontal distance from zero (meters)
Zero 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Lamp size (watts): 60
75
100
150
200
P
H
P
H
P
H
P
H
P
H
80 52 37 24 16 10 6
80 50 32 17 10 6 2
100 66 44 26 15 10 6
100 62 36 20 9 5 2
160 110 68 42 26 18 13 4
160 100 52 28 16 10 6 2
250 180 120 80 48 32 22 11 4
250 170 100 60 30 16 10 4 1
260 240 160 96 58 40 28 20 15 I0
260 220 130 62 30 19 12 6 4 2
pended 2 meters high and are 4 meters apart, the maximum light that is available at a point equidistant between the lamps is 104 lux, i.e. 52 lux from each bulb from the perpendicular (P) value in Table IIIb. Light readings from the "H" column (Table IIlb) for these same conditions would yield only 76 lux (38 + 38).
267 TABLE IIIa Light values in lux for various size lamps without reflectors placed 2. 0 meters above the soil surface Horizontal distance from zero (meters)
zero 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Lamp size (watts): 60
75
P
H
13 12 10 8 6 5
13 tl 9 6 4 3
P 17 15 13 11 8 6 5
100
150
200
H
P
H
P
H
P
H
17 14 12 8 5 4 2
26 22 18 15 11 8 6
26 22 16 11 8 5 3
46 40 34 28 21 17 12 10 8 6
46 40 32 22 14 11 6 5 4 2
60 56 44 34 25 20 15 11 9 7
60 54 40 28 16 13 8 6 4 2
TABLE IIIb Light values in lux for various size lamps with a 30-era-diameter white dome reflector placed 2.0 meters above the soil surface Horizontal distance from zero (meters)
zero 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
Lamp size (watts): 60
75
100
150
200
P
H-
P
H
P
H
P
H
P
H
42 34 26 19 14 10 6 5
42 34 24 16 10 7 3 2
56 40 32 22 15 10 7 6
56 40 28 17 10 6 4 2
80 68 50 37 26 20 14 9 7 5
80 64 43 29 18 12 6 4 3 2
140 120 86 64 44 33 24 17 13 6 4
140 120 80 52 32 20 13 8 6 3 2
160 140 120 82 52 41 30 22 18 14 12 8
160 140 110 66 38 26 15 11 8 6 4 2
If a large chrysanthemum area is to be lighted allowing a minimum of 100 lux, large lamps placed high above the plants are most desirable to obtain even light distributions providing there are no ceiling limitations as in a greenhouse. After the desired lamp heights and wattages are chosen, one must calculate the distance needed between lamps to obtain the
268 TABLE IV The variation in light intensity (lux) from a 150-watt lamp as affected by changes in line voltage from 225 to 185 volts. The lamp, with or without reflector, was placed 2.0 meters above the soil surface. All values were recorded to obtain the maximum reading at each position. Horizontal distance from zero (meters)
zero 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
No reflector
30-cm-diameter reflector
225 V P
205 V P
185 V P
225 V P
205 V P
185 V P
46 40 34 28 21 17 12 10 8 6
32 28 24 18 14 11 8 6
21 17 14 13 10 8 6
140 120 86 64 44 33 24 17 13 6 4
100 80 57 44 31 22 17 12 10 4
65 58 42 32 22 17 12 8 7
minimum 100 lux. Although the light from each lamp is additive, t h e lowest light flux density in the middle o f a large field o f lamps exists at a point equidistant from four lamps. The light which is emitted from lamps adjacent to the four lamps in question is almost negligible because o f the large distances and the low angles o f light. When comparing Tables IIIa and IIIb, it is evident that the light flux density is greatest when reflectors are used. In our calculation example, we shall place 200-watt lamps with reflectors at a 2-meter height (Table IIIb). Since a point equidistant from four lamps has the lowest light flux density, it is desirable to adjust that central area first to about 100 lux. Using the "H" column o f Table IIIb, one finds 26 lux at 2.5 meters away from any o n e lamp, but from four lamps, the total value becomes 104 lux (4 × 26). The direct horizontaI distance from the center to any one lamp is n o w established at 2.5 meters. These distances form 2 sides Of an isosceles triangle and the distance between lamps (base o f triangle) can be calculated by.multiplying 2.5 meters by ~ (approximately 1.4) which is equal to 3.5 meters. This calculation is derived from: Theorem of Pythagoras: a 2 + b 2 = c 2 The simple rule (for a square spacing) is always to multiply the distance from the center to any one lamp by 1.4 to obtain the distance between
269 lamps. This should be done after calculating the desired minimum fight flux d e n s i t y at a point which is equidistant from the four lamps, i.e. the area o f least light. In the above calculation the light flux density at the midpoint between two lamps spaced 3.5 meters apart would be 104 lux. The value o f 52 lux from each lamp at a distance of 1.75 meters to a median point is interpolated from values of 66 and 38 lux at 1.5 and 2.0 meters, respectively. Thus the minimum light flux density o f 100 lux prescribed for chrysanthemums has been fulfilled at all points within the square design from " H " values in Table IIIb. Lamp spacings can also be calculated by laying out the distances to scale on graph paper, measuring the points desired and determining the light value at any location from the proper table. The light flux density at any one point is the summation o f all light values from all sources that reach that point. Lamps m a y be spaced in ways other than a square, such as a rectangle or rhombus, however, central points of these configurations are difficult to determine. When calculating light emission from a single line of lamps or on the edge of a large installation, one should be aware that some o f the plants will only be illuminated from one direction. As the plants on the outside row will not be illuminated evenly, the outside row of lamps should be placed relatively close to the edge o f the field o f plants to compensate for the unidirectional fight. The readings, which appear in Tables I, II and III, were measured with 225 volts. The light emitted from each lamp under these conditions was rated at 100%. When the line voltage drops from 225 to 205 or to 185 (Table IV), the light emitted b y the lamp is proportionately less. In practice the line voltage may drop because o f high resistance due to the wire being t o o fine, or too long, overloaded or having high-resistance connections. Some or all o f these in combination m a y contribute to a voltage drop which results in reduced light emission from each lamp. R e d u c e d light emission may also result from the accumulation of dust on the lamps or the reflectors. Care should be taken to clean these surfaces periodically.
TABLES FOR CALCULATING DESIRED LIGHT FLUX DENSITIES FOR HORTICULTURAL CROPS
ANTON M. KOFRANEK* and MICHAEL ROBINSON
Department o f Ornamental Horticulture, The Hebrew University, Rehovot (Israel) *Permanent address: Department of Environmental Horticulture, University of California, Davis, Calif. 95616 (U.S.A.) (Received February 9th, 1973)
Light emissions in lux from single incandescent lamps ranging from 60 to 200 watts were arranged in tables according to the lamp height above the ground. The method of calculating the desired lamp spacing to achieve a desired light intensity is illustrated. A table showing the reduced light emission due to various voltage drops is given along with precautions to avoid them.
INTRODUCTION The photoperiodic requirements of many plants are well known, but very often it is not known how to space lamps to achieve the desired light flux densities to induce or prevent flowering. There is general agreement that incandescent light of relatively low light intensity may be ample to trigger a photoperiodic response. It is possible to calculate light flux densities by known formulae or to measure them empirically with sophisticated devices, but these tables are intended to aid those who wish to calculate a desired minimum light level without resorting to formulae or light meters. Therefore, these tables may be useful to growers, advisory service personnel or the horticultural scientist wishing to calculate the proper incandescent lamp spacing for purposes of supplying supplementary light to a certain crop. METHODS Light readings in lux were recorded with a Gossen light meter from single incandescent lamps suspended 1.0, 1.5, or 2.0 meters above the soil.
264 Readings were made with the light sensing device placed perpendicular (P) to the light source or with the device placed horizontally (H) on the soil surface. Light readings were made on a flat surface, in the dark location without any side reflective surfaces, at horizontal intervals o f 0.5 meter distant from the lamp until the readings were about 3 lux or less. Lamps o f clear glass o f various wattage ratings shown in the tables were used with or without a white dome reflector. The light measurements from each lamp source were assembled in the tables and arranged in relation to the lamp height above the ground, and the distance, starting from a point directly beneath the lamp (zero), measured away from zero in a horizontal direction. HOW TO USE THE TABLES First one must decide what minimun light flux density is necessary for the crop. F o r example, the chrysanthemum, a short-day plant, requires a minimum of 100 lux at 15 °C night temperature to inhibit premature flower bud initiation. Therefore, one must calculate the spacing o f the lamps to record 100 lux in the darkest location o f the glasshouse or field. The tables clearly illustrate that large lamps with or without reflectors, spaced high above the soil surface emit light further than smaller lamps. Note that there are t w o different lux values listed under each bulb size. The " P " value was recorded when the light sensing device was held perpendicular to the light source to give a m a x i m u m reading at a given distance. The " H " value was obtained with the device held flat on the ground at all points, and much o f the light was reflected o f f the glass surface of the sensing device at distant points. One can expect that the plant perceives light somewhere between these two values (H and P) depending on the position and orientation of the leaves, the amount of shading from other plants and the d i s t a n c e from the light source. If a dense canopy is to be lighted only at the plant tops, then the readings from the " H " column should be chosen. On the other hand, if the crop is sparsely planted and the light is able to penetrate at many angles, then use of the " P " column would be more appropriate. The decision as to which column should be used depends mainly on the growth habit o f the crop. If only a single bed is to be lighted in a greenhouse, one should choose small lamps placed close to the plants in order to confine the light to a narrow area. If one desires illumination o f the entire greenhouse or a large field, then it is wise to choose the 200-watt lamps suspended 2.0 meters above the growing surface for the most efficient light distribution. The light readings from t w o or more light sources u p o n one point are additive. For example, if t w o 200-watt lamps with reflectors are sus-
265 TABLES I, II, III The horizontal distance on the ground was measured from a point starting directly beneath the lamp (zero) and continuing in a straight line at 0.5 meter intervals. The "P" value was recorded with the sensing device always perpendicular to the light source (greatest possible reading) and the " H " value was recorded with the device laid horizontal on the soil surface at all distances.
TABLE Ia Light values in lux for various size lamps without reflectors placed 1.0 meter above the soil surface Horizontal distance from zero (meters)
zero 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Lamp size (watts): 60
75
100
150
200
P
H
P
H
P
H
P
H
P
H
50 40 23 14 8 6
50 30 15 7 4 2
60 46 32 18 12 8 6
60 42 22 10 5 3 2
92 46 42 25 17 12 8
92 58 30 13 6 4 2
180 120 76 48 27 19 14 11
180 100 52 24 9 7 3 2
200 180 120 60 38 27 19 15 11 9
200 140 76 30 18 11 7 4 3 2
TABLE Ib Light values in lux for various size lamps with a 30-cm-diameter white dome reflector placed 1.0 meter above the soil surface Horizontal distance from zero (meters)
zero 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Lamp size (watts): 60
75
100
150
200
P
H
P
H
P
H
P
H
P
H
160 90 50 26 14 4
160 70 40 14 6 2
280 120 58 26 10 4
280 100 40 13 5 1
340 200 93 44 22 5
340 180 62 20 10 2
500 310 160 76 36 11 5
560 250 100 42 18 6 1
570 400 210 100 54 38 24 11
570 330 140 50 22 14 5 2
266
•
TABLE IIa Light values in lux for various size lamps without reflectors placed 1.5 meters above the soil surfac.e Horizontal distance from zero (meters)
zero 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Lamp size (watts): 60
75
100
150
200
P
H
P
H
P
H
P
H
P
H
24 19 15 12 9 6
24 18 13 8 5 3
30 26 19 13 9 6 5
30 25 15 10 5 3 2
42 36 30 19 14 11 7
42 34 24 13 8 5 3
74 66 50 36 25 18 14 12 9
74 62 42 26 14 9 6 4 3
100 86 64 46 28 22 16 12 9 7
100 80 54 34 20 11 7 4 3 2
TABLE IIb Light values in lux for various size lamps with a 30-era-diameter white dome reflector placed 1.5 meters above the soil surface Horizontal distance from zero (meters)
Zero 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Lamp size (watts): 60
75
100
150
200
P
H
P
H
P
H
P
H
P
H
80 52 37 24 16 10 6
80 50 32 17 10 6 2
100 66 44 26 15 10 6
100 62 36 20 9 5 2
160 110 68 42 26 18 13 4
160 100 52 28 16 10 6 2
250 180 120 80 48 32 22 11 4
250 170 100 60 30 16 10 4 1
260 240 160 96 58 40 28 20 15 I0
260 220 130 62 30 19 12 6 4 2
pended 2 meters high and are 4 meters apart, the maximum light that is available at a point equidistant between the lamps is 104 lux, i.e. 52 lux from each bulb from the perpendicular (P) value in Table IIIb. Light readings from the "H" column (Table IIlb) for these same conditions would yield only 76 lux (38 + 38).
267 TABLE IIIa Light values in lux for various size lamps without reflectors placed 2. 0 meters above the soil surface Horizontal distance from zero (meters)
zero 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Lamp size (watts): 60
75
P
H
13 12 10 8 6 5
13 tl 9 6 4 3
P 17 15 13 11 8 6 5
100
150
200
H
P
H
P
H
P
H
17 14 12 8 5 4 2
26 22 18 15 11 8 6
26 22 16 11 8 5 3
46 40 34 28 21 17 12 10 8 6
46 40 32 22 14 11 6 5 4 2
60 56 44 34 25 20 15 11 9 7
60 54 40 28 16 13 8 6 4 2
TABLE IIIb Light values in lux for various size lamps with a 30-era-diameter white dome reflector placed 2.0 meters above the soil surface Horizontal distance from zero (meters)
zero 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
Lamp size (watts): 60
75
100
150
200
P
H-
P
H
P
H
P
H
P
H
42 34 26 19 14 10 6 5
42 34 24 16 10 7 3 2
56 40 32 22 15 10 7 6
56 40 28 17 10 6 4 2
80 68 50 37 26 20 14 9 7 5
80 64 43 29 18 12 6 4 3 2
140 120 86 64 44 33 24 17 13 6 4
140 120 80 52 32 20 13 8 6 3 2
160 140 120 82 52 41 30 22 18 14 12 8
160 140 110 66 38 26 15 11 8 6 4 2
If a large chrysanthemum area is to be lighted allowing a minimum of 100 lux, large lamps placed high above the plants are most desirable to obtain even light distributions providing there are no ceiling limitations as in a greenhouse. After the desired lamp heights and wattages are chosen, one must calculate the distance needed between lamps to obtain the
268 TABLE IV The variation in light intensity (lux) from a 150-watt lamp as affected by changes in line voltage from 225 to 185 volts. The lamp, with or without reflector, was placed 2.0 meters above the soil surface. All values were recorded to obtain the maximum reading at each position. Horizontal distance from zero (meters)
zero 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
No reflector
30-cm-diameter reflector
225 V P
205 V P
185 V P
225 V P
205 V P
185 V P
46 40 34 28 21 17 12 10 8 6
32 28 24 18 14 11 8 6
21 17 14 13 10 8 6
140 120 86 64 44 33 24 17 13 6 4
100 80 57 44 31 22 17 12 10 4
65 58 42 32 22 17 12 8 7
minimum 100 lux. Although the light from each lamp is additive, t h e lowest light flux density in the middle o f a large field o f lamps exists at a point equidistant from four lamps. The light which is emitted from lamps adjacent to the four lamps in question is almost negligible because o f the large distances and the low angles o f light. When comparing Tables IIIa and IIIb, it is evident that the light flux density is greatest when reflectors are used. In our calculation example, we shall place 200-watt lamps with reflectors at a 2-meter height (Table IIIb). Since a point equidistant from four lamps has the lowest light flux density, it is desirable to adjust that central area first to about 100 lux. Using the "H" column o f Table IIIb, one finds 26 lux at 2.5 meters away from any o n e lamp, but from four lamps, the total value becomes 104 lux (4 × 26). The direct horizontaI distance from the center to any one lamp is n o w established at 2.5 meters. These distances form 2 sides Of an isosceles triangle and the distance between lamps (base o f triangle) can be calculated by.multiplying 2.5 meters by ~ (approximately 1.4) which is equal to 3.5 meters. This calculation is derived from: Theorem of Pythagoras: a 2 + b 2 = c 2 The simple rule (for a square spacing) is always to multiply the distance from the center to any one lamp by 1.4 to obtain the distance between
269 lamps. This should be done after calculating the desired minimum fight flux d e n s i t y at a point which is equidistant from the four lamps, i.e. the area o f least light. In the above calculation the light flux density at the midpoint between two lamps spaced 3.5 meters apart would be 104 lux. The value o f 52 lux from each lamp at a distance of 1.75 meters to a median point is interpolated from values of 66 and 38 lux at 1.5 and 2.0 meters, respectively. Thus the minimum light flux density o f 100 lux prescribed for chrysanthemums has been fulfilled at all points within the square design from " H " values in Table IIIb. Lamp spacings can also be calculated by laying out the distances to scale on graph paper, measuring the points desired and determining the light value at any location from the proper table. The light flux density at any one point is the summation o f all light values from all sources that reach that point. Lamps m a y be spaced in ways other than a square, such as a rectangle or rhombus, however, central points of these configurations are difficult to determine. When calculating light emission from a single line of lamps or on the edge of a large installation, one should be aware that some o f the plants will only be illuminated from one direction. As the plants on the outside row will not be illuminated evenly, the outside row of lamps should be placed relatively close to the edge o f the field o f plants to compensate for the unidirectional fight. The readings, which appear in Tables I, II and III, were measured with 225 volts. The light emitted from each lamp under these conditions was rated at 100%. When the line voltage drops from 225 to 205 or to 185 (Table IV), the light emitted b y the lamp is proportionately less. In practice the line voltage may drop because o f high resistance due to the wire being t o o fine, or too long, overloaded or having high-resistance connections. Some or all o f these in combination m a y contribute to a voltage drop which results in reduced light emission from each lamp. R e d u c e d light emission may also result from the accumulation of dust on the lamps or the reflectors. Care should be taken to clean these surfaces periodically.