Seasoned lamps, operated at constant voltage, gradually decrease in color temperature because of evaporation of the filament. Since the current does not decrease enough to correspond to an appreciable change in filament temperature, this decrease in color temperature is ascribed in large part to the accumulation of the familiar brown film on the inside surface of the bulb. The size and shape of the bulb is, therefore, important; each type must be separately investigated. Results are given for five types. The effect of operating seasoned lamps is a gradual decrease in color temperature, linear with time, which, like the rate of evaporation of tungsten, was found for 400-watt projection lamps, to be nearly proportional to the thirtieth power of the temperature. The effect of seasoning new lamps for one hour at about rated voltage was a rise in color temperature from 3050° to about 3140°K, approximately half of which occurred in the first three minutes.
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The types of lamp investigated, their number, duration of test, and color temperature.
Rated Watts
Type of Bulb
Type of Filament
Number Tested
Duration of Test, Hours
Type of Current
Initial Color Temperature
Test Started
400
Tubular, gas-filled
Monoplane
5
64
A.C.
3100°K
6/18/32
5
64
D.C.
3100
6/18/32
5
700
A.C.
2850
8/ 3/32
5
700
A.C.
2360
8/ 3/32
500
Pear-shaped, gas-filled
Crown (C7)
5
128
D.C.
2850
5/ 4/31
500
Pear-shaped, gas-filled
Circular-coil (C7A)
5
128
D.C.
2840
10/19/31
60
Pear-shaped, vacuum
Squirrel-cage
5
128
D.C.
2360
5/ 4/31
60
Pear-shaped, vacuum
Circular-coil (C9)
5
128
D.C.
2360
10/19/31
Table II
Change in color temperature according to distribution of tungsten removed from filament. (These computations refer to a one percent decrease in current, I″/I0=0.99.)
Fraction of Original Length, L, Reduced in Diameter f
Fraction of Original Diameter Required Over Fractional Length, f, to Produce a 1% Decrease in Current see Eq. (14)c
Fractional Increase in Color Temperature at Narrow Part of Filament θ″/θ0=0.994*/c0.9θ″/θ0
Color Temperature of Unchanged Part of Filament, θ′, of Narrow Part, θ″, and of Whole Filament, θR/B for Two Initial Color Temperatures, θ0, in Degrees Kelvin
The factor, 0.994 is introduced because Eq. (3) requires θ″/θ0 to equal 0.999 for c=0.995; this is a drop in color temperature with voltage and length constant due to decreased current; it does not follow from Eq. (9) which refers to constant current.
The length of filament, (1−f)L, unchanged in diameter has a color temperature lower than θ0 because of the one percent decrease in current, thus: from Eq. (12): θ′/θ0=(I′/I0)0.60=0.990.60=0.994.
Table III
Current, luminous efficiency and color temperature for the types of lamp studied, and their time rates of change when the lamps are operated at constant voltage V; see Table I for further description of lamps.
Identification of Lamp Type
Voltage V
Current I0 Amperes
dI/dt Milliamperes per Hour
Luminous Efficiency ∊0 Lumens per Watt
d∊/dt Lumens per Watt per Hour
Color Temperature θ0 Degrees Kelvin
dθ/dt Degrees Kelvin per Hour
Projection
110
3.82
−0.6
24.6
−0.058 ±0.007
3140
−0.6 ±0.1
85
3.31
−0.03
16.0
−0.0026 ± .0004
2850
−0.06 ± .01
52
2.54
−0.003
5.7
−0.00015± .00011
2360
−0.006± .005
Old Standard Type (C7)
110
4.20
+0.04
16.5
−0.0029 ± .0011
2850
−0.09 ± .05
New Standard Type (C7A)
100
4.08
+0.02
16.1
−0.0026 ± .0011
2840
−0.08 ± .06
Commercial Vacuum Type
105
0.475
−0.01
7.4
−0.0018 ± .0009
2360
−0.01 ± .08
Standard Vacuum Type
95
0.469
0.00
7.4
−0.0015 ± .0005
2360
−0.05 ± .04
Table IV
Dependence on temperature of filament of rate of decline of luminous efficiency, ∊, and color temperature, θ, of the lamp.
Taken from data by Forsythe and Worthing (see reference 1) connecting true temperature with color temperature on the Nela scale.
Table V
Alternating-current operation compared to direct-current operation for 400-watt projection lamps at rated voltage (110 volts, 3140°K) burned base down.
Type of Operation
d∊/dt L/W per Hour
dθ/dt Degrees K per Hour
A.C.
−0.058±0.006
−0.33 ±0.18
D.C.
−0.059± .006
−0.69± .12
Table VI
Spectral transmittance of brown deposit on the bulb of a lamp whose color temperature had declined about 20°K during a 64-hour period of burning near 3140°K; illuminating beam and direction of view both nearly normal to surfaces of deposit.
Wave-Length
Transmittance
Seventh Root of Transmittance at Top of Bulb
In Line of Sight
At Top of Bulb
440
0.848
0.306
0.843
480
.861
.354
.861
520
.874
.400
.876
560
.886
.437
.888
600
.897
.471
.897
640
.907
.502
.906
680
.916
.533
.914
Tables (6)
Table I
The types of lamp investigated, their number, duration of test, and color temperature.
Rated Watts
Type of Bulb
Type of Filament
Number Tested
Duration of Test, Hours
Type of Current
Initial Color Temperature
Test Started
400
Tubular, gas-filled
Monoplane
5
64
A.C.
3100°K
6/18/32
5
64
D.C.
3100
6/18/32
5
700
A.C.
2850
8/ 3/32
5
700
A.C.
2360
8/ 3/32
500
Pear-shaped, gas-filled
Crown (C7)
5
128
D.C.
2850
5/ 4/31
500
Pear-shaped, gas-filled
Circular-coil (C7A)
5
128
D.C.
2840
10/19/31
60
Pear-shaped, vacuum
Squirrel-cage
5
128
D.C.
2360
5/ 4/31
60
Pear-shaped, vacuum
Circular-coil (C9)
5
128
D.C.
2360
10/19/31
Table II
Change in color temperature according to distribution of tungsten removed from filament. (These computations refer to a one percent decrease in current, I″/I0=0.99.)
Fraction of Original Length, L, Reduced in Diameter f
Fraction of Original Diameter Required Over Fractional Length, f, to Produce a 1% Decrease in Current see Eq. (14)c
Fractional Increase in Color Temperature at Narrow Part of Filament θ″/θ0=0.994*/c0.9θ″/θ0
Color Temperature of Unchanged Part of Filament, θ′, of Narrow Part, θ″, and of Whole Filament, θR/B for Two Initial Color Temperatures, θ0, in Degrees Kelvin
The factor, 0.994 is introduced because Eq. (3) requires θ″/θ0 to equal 0.999 for c=0.995; this is a drop in color temperature with voltage and length constant due to decreased current; it does not follow from Eq. (9) which refers to constant current.
The length of filament, (1−f)L, unchanged in diameter has a color temperature lower than θ0 because of the one percent decrease in current, thus: from Eq. (12): θ′/θ0=(I′/I0)0.60=0.990.60=0.994.
Table III
Current, luminous efficiency and color temperature for the types of lamp studied, and their time rates of change when the lamps are operated at constant voltage V; see Table I for further description of lamps.
Identification of Lamp Type
Voltage V
Current I0 Amperes
dI/dt Milliamperes per Hour
Luminous Efficiency ∊0 Lumens per Watt
d∊/dt Lumens per Watt per Hour
Color Temperature θ0 Degrees Kelvin
dθ/dt Degrees Kelvin per Hour
Projection
110
3.82
−0.6
24.6
−0.058 ±0.007
3140
−0.6 ±0.1
85
3.31
−0.03
16.0
−0.0026 ± .0004
2850
−0.06 ± .01
52
2.54
−0.003
5.7
−0.00015± .00011
2360
−0.006± .005
Old Standard Type (C7)
110
4.20
+0.04
16.5
−0.0029 ± .0011
2850
−0.09 ± .05
New Standard Type (C7A)
100
4.08
+0.02
16.1
−0.0026 ± .0011
2840
−0.08 ± .06
Commercial Vacuum Type
105
0.475
−0.01
7.4
−0.0018 ± .0009
2360
−0.01 ± .08
Standard Vacuum Type
95
0.469
0.00
7.4
−0.0015 ± .0005
2360
−0.05 ± .04
Table IV
Dependence on temperature of filament of rate of decline of luminous efficiency, ∊, and color temperature, θ, of the lamp.
Taken from data by Forsythe and Worthing (see reference 1) connecting true temperature with color temperature on the Nela scale.
Table V
Alternating-current operation compared to direct-current operation for 400-watt projection lamps at rated voltage (110 volts, 3140°K) burned base down.
Type of Operation
d∊/dt L/W per Hour
dθ/dt Degrees K per Hour
A.C.
−0.058±0.006
−0.33 ±0.18
D.C.
−0.059± .006
−0.69± .12
Table VI
Spectral transmittance of brown deposit on the bulb of a lamp whose color temperature had declined about 20°K during a 64-hour period of burning near 3140°K; illuminating beam and direction of view both nearly normal to surfaces of deposit.