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Lupine Betty R Color Rendition for Photography
The Lupine Betty R lamphead can be used for photography, e.g. at night in particular.
Color temperature is daylight at ~ 5500° Kelvin, but there is a green cast that needs correction of roughly +40 magenta in Adobe Camera RAW, as shown at right, though this can be less at lower power settings.
Balancing the tint
With night shooting, filtering the light would be preferable because the color correction applies only to the subject matter that is illuminated by the lamphead. The Betty R lamphead is ~52mm in diameter, so it should be possible to tape a 52mm magenta filter over the lamphead. Except that no such filter seems to exist, but see below for tests using gel filters, would could be cut and taped over the front.
Ideally, Lupine would offer a replacement lens that corrects for the green cast, instead of the standard clear glass one. This would provide superior color rendition to the eye at night as well, though at the cost of some brightness loss— but visual acuity is sensitive to both color balance and brightness.
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Correcting the color
These tests were run using a set of Lee 4x4" Magenta Color Compensating Polyester Filter Set (CC05M, CC10M, CC20M, CC25M). The gel filters were placed over the light source and a series with different filtration was shot.
Color temperature (blue/yellow) ranged from 5350°K at 12W to 5500°K at 40W, but no effort was made to determine if this changed as the lamphead warmed.
It was quickly seen that +15 CC magenta is too much magenta. The proper correction is either a +5M or +10 filter gel filter over the light source, depending on power level :
- At 12 watts, +10 CC magenta is almost perfect, using a color temperature of 5350° K in ACR.
- At 40 watts (maximum) the remains slightly greenish, needing than 10 CC magenta correction, but more than 5 CC magenta. Color temperature of 5550° K in ACR.
The various intensity levels, ambient temperature, actual temperature of the LEDs within the lamphead, variation among units and their individual LEDs, etc all mean that actual correction could vary slightly by lamphead and actual usage. Hence anything within ~5 CC magenta is about as close as one might hope for under field conditions.
Mouse over the series to see the differences, click for larger.
Tint is just about perfect with +10 CC magenta filtration.
Tint neutrality improves at the 40W setting. The necessary filtration is between +5M (slightly green) and +10M (slightly magenta). The ideal filter would be somewhere in between.