Optics and Radiometry Laboratory, School of Optometry and Vision Science, University of New South Wales, Sydney, NSW 2052, Australia (email: s.dain@unsw.edu.au)
The Farnsworth Lantern has been accepted in many occupational applications. Modern instruments purporting to be equivalent were inevitable given the ubiquity of its use. The OPTEC900® (Stereo Optical) has been validated and adopted as an acceptable substitute, although the fail rate is slightly higher. The Farnsworth Flashlight (Gulden Ophthalmics) has been marketed without validation. The color and luminous intensities of the three lanterns’ stimuli were measured using a Topcon SR-3 telespectroradiometer and compared with Farnsworth’s recommendations. The OPTEC900® chromaticities comply with Farnsworth’s recommendations, but the luminous intensities are rather different. The Farnsworth Flashlight differs from the Farnsworth Lantern in both chromaticity and luminous intensities. A cheap and portable equivalent to the Farnsworth Lantern is a highly attractive product. However, the Farnsworth Flashlight is significantly different in construction from the Farnsworth Lantern.
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Chromaticities and Luminous Intensities of the 18 Colors in Each Lantern in CIE 1931 , Color Space
FALANT
OPTEC900
Flashlight
Position
Color
Luminous intensity (mcd)
Luminous intensity (mcd)
Luminous intensity (mcd)
Top
1
Green
0.217
0.694
0.36
0.190
0.711
0.86
0.238
0.633
2.28
2
White
0.483
0.416
0.51
0.473
0.412
1.51
0.480
0.442
0.86
3
Green
0.219
0.700
0.33
0.191
0.709
1.11
0.238
0.633
1.97
4
Green
0.220
0.695
0.25
0.194
0.694
0.30
0.239
0.621
1.05
5
Red
0.630
0.310
0.39
0.622
0.293
0.92
0.676
0.317
0.45
6
White
0.479
0.418
0.30
0.468
0.405
0.80
0.487
0.440
0.42
7
White
0.484
0.417
0.55
0.471
0.411
1.96
0.481
0.442
0.88
8
Red
0.630
0.310
0.31
0.619
0.293
0.72
0.677
0.317
0.44
9
Red
0.629
0.307
0.34
0.620
0.293
0.96
0.677
0.316
0.44
Bottom
1
Red
0.635
0.311
0.97
0.621
0.293
1.09
0.675
0.317
0.58
2
Green
0.224
0.699
0.44
0.193
0.697
0.59
0.240
0.620
1.25
3
White
0.485
0.418
0.76
0.469
0.404
1.06
0.486
0.439
0.55
4
Green
0.246
0.684
1.48
0.194
0.707
1.32
0.240
0.633
2.40
5
Green
0.228
0.698
0.81
0.191
0.709
1.09
0.240
0.632
2.35
6
Red
0.637
0.313
2.06
0.622
0.288
1.60
0.683
0.313
1.95
7
White
0.486
0.418
0.77
0.470
0.405
0.87
0.487
0.439
0.50
8
White
0.493
0.417
1.75
0.472
0.412
2.23
0.482
0.441
1.01
9
Red
0.636
0.304
1.83
0.622
0.289
1.58
0.683
0.313
1.89
Table 2.
Chromaticity Difference between the Mean Red, Green, and White Lights of the Flashlight, FALANT, and OPTEC900 Calculated in CIE Spacea
Red-Green
Red-White
Green-White
Protan
Deutan
Protan
Deutan
Protan
Deutan
FALANT
0.387
0.387
0.387
0.191
0.191
0.191
0.197
0.196
0.197
OPTEC 900
0.410
0.408
0.410
0.204
0.203
0.204
0.206
0.205
0.206
Flashlight
0.407
0.407
0.406
0.236
0.236
0.236
0.171
0.171
0.170
This is inversely representative of the difficulty of the task for those with normal color vision. The chromaticity differences for protans and deutans are the distances along the dichromatic confusion lines. This is a measure of the ease with which congenital protan and deutan observers will be able to distinguish the colors.
Table 3.
Lightness () Difference between the Mean Red, Green, and White Lights of the Flashlight, FALANT, and OPTEC900a
Red-Green
Red-White
Green-White
FALANT
10.5
3.8
OPTEC 900
8.8
Flashlight
4.7
29.7
This is a measure of the achromatic difference between colors of a pair, and the OPTEC900 and Flashlight need to match the difference in the FALANT colors to be a faithful replication.
Table 4.
Residual Chromaticity Difference for Congenital Dichromatsa
Red-Green
Red-White
Green-White
Protan
Deutan
Protan
Deutan
Protan
Deutan
Flashlight
0.007
0.026
0.008
0.019
0.002
0.009
FALANT
0.022
0.000
0.008
0.009
0.020
0.008
OPTEC900
0.038
0.004
0.020
0.007
0.025
0.010
The chromaticity difference calculated in the direction perpendicular to the dichromatic confusion lines, i.e., the color difference still available to congenital protanopes and deuteranopes.
Table 5.
Difference between the Angle of the Line Joining the Mean Colors and the Dichromatic Confusion Lines as a Measure of Alignment on the Dichromatic Confusion Linesa
Red-Green
Red-White
Green-White
Protan angle error°
OPTEC900
7
FALANT
Flashlight
1
2
Deutan angle error°
FALANT
0
3
OPTEC900
2
Flashlight
4
4
3
To be faithful replications of the FALANT, the OPTEC 900 and the Flashlight should show the same values.
Tables (5)
Table 1.
Chromaticities and Luminous Intensities of the 18 Colors in Each Lantern in CIE 1931 , Color Space
FALANT
OPTEC900
Flashlight
Position
Color
Luminous intensity (mcd)
Luminous intensity (mcd)
Luminous intensity (mcd)
Top
1
Green
0.217
0.694
0.36
0.190
0.711
0.86
0.238
0.633
2.28
2
White
0.483
0.416
0.51
0.473
0.412
1.51
0.480
0.442
0.86
3
Green
0.219
0.700
0.33
0.191
0.709
1.11
0.238
0.633
1.97
4
Green
0.220
0.695
0.25
0.194
0.694
0.30
0.239
0.621
1.05
5
Red
0.630
0.310
0.39
0.622
0.293
0.92
0.676
0.317
0.45
6
White
0.479
0.418
0.30
0.468
0.405
0.80
0.487
0.440
0.42
7
White
0.484
0.417
0.55
0.471
0.411
1.96
0.481
0.442
0.88
8
Red
0.630
0.310
0.31
0.619
0.293
0.72
0.677
0.317
0.44
9
Red
0.629
0.307
0.34
0.620
0.293
0.96
0.677
0.316
0.44
Bottom
1
Red
0.635
0.311
0.97
0.621
0.293
1.09
0.675
0.317
0.58
2
Green
0.224
0.699
0.44
0.193
0.697
0.59
0.240
0.620
1.25
3
White
0.485
0.418
0.76
0.469
0.404
1.06
0.486
0.439
0.55
4
Green
0.246
0.684
1.48
0.194
0.707
1.32
0.240
0.633
2.40
5
Green
0.228
0.698
0.81
0.191
0.709
1.09
0.240
0.632
2.35
6
Red
0.637
0.313
2.06
0.622
0.288
1.60
0.683
0.313
1.95
7
White
0.486
0.418
0.77
0.470
0.405
0.87
0.487
0.439
0.50
8
White
0.493
0.417
1.75
0.472
0.412
2.23
0.482
0.441
1.01
9
Red
0.636
0.304
1.83
0.622
0.289
1.58
0.683
0.313
1.89
Table 2.
Chromaticity Difference between the Mean Red, Green, and White Lights of the Flashlight, FALANT, and OPTEC900 Calculated in CIE Spacea
Red-Green
Red-White
Green-White
Protan
Deutan
Protan
Deutan
Protan
Deutan
FALANT
0.387
0.387
0.387
0.191
0.191
0.191
0.197
0.196
0.197
OPTEC 900
0.410
0.408
0.410
0.204
0.203
0.204
0.206
0.205
0.206
Flashlight
0.407
0.407
0.406
0.236
0.236
0.236
0.171
0.171
0.170
This is inversely representative of the difficulty of the task for those with normal color vision. The chromaticity differences for protans and deutans are the distances along the dichromatic confusion lines. This is a measure of the ease with which congenital protan and deutan observers will be able to distinguish the colors.
Table 3.
Lightness () Difference between the Mean Red, Green, and White Lights of the Flashlight, FALANT, and OPTEC900a
Red-Green
Red-White
Green-White
FALANT
10.5
3.8
OPTEC 900
8.8
Flashlight
4.7
29.7
This is a measure of the achromatic difference between colors of a pair, and the OPTEC900 and Flashlight need to match the difference in the FALANT colors to be a faithful replication.
Table 4.
Residual Chromaticity Difference for Congenital Dichromatsa
Red-Green
Red-White
Green-White
Protan
Deutan
Protan
Deutan
Protan
Deutan
Flashlight
0.007
0.026
0.008
0.019
0.002
0.009
FALANT
0.022
0.000
0.008
0.009
0.020
0.008
OPTEC900
0.038
0.004
0.020
0.007
0.025
0.010
The chromaticity difference calculated in the direction perpendicular to the dichromatic confusion lines, i.e., the color difference still available to congenital protanopes and deuteranopes.
Table 5.
Difference between the Angle of the Line Joining the Mean Colors and the Dichromatic Confusion Lines as a Measure of Alignment on the Dichromatic Confusion Linesa
Red-Green
Red-White
Green-White
Protan angle error°
OPTEC900
7
FALANT
Flashlight
1
2
Deutan angle error°
FALANT
0
3
OPTEC900
2
Flashlight
4
4
3
To be faithful replications of the FALANT, the OPTEC 900 and the Flashlight should show the same values.