How to dim a light without changing its color temperature

Chan Tran · Jun 3, 2017

I need a light source to test camera exposure meter as well as testing camera shutter speeds on auto.
I used to use a Beseler computerized 45 dichoic head for this. I just dial in equal filters to create neutral density.
I am thinking of a few ways.
1. Copying the dichroic head design using a mechanism that moves the filter in and out of the light path but instead of using filter I would use just an opaque gate to reduce the amount of light.
2. Inserting ND filter in the light path.

Dimming the light by reducing power would lower the color temperature.
Any ideas?

~andi · Jun 3, 2017

I think a LED light-source can be dimmed without changing the color temperature.

I'm using a ND filter on my enlarger for small prints. Works fine.

Andi

jlbruyelle · Jun 3, 2017

As an alternative to a ND filter, the Ahel colour head uses a metal plate with drilled holes in the light path between the halogen bulb and the diffusor box. A hand-actuated slide allows to insert the plate or to remove it from the light path - intermediate positions are possible and don't cause irregular lighting, owing to the location of the plate. Three advantages: a metal plate does not change the colour of the light, it does not age with heat, and it is easy to control accurately the light reduction factor by the size and density of the holes (providing you have accurate machining equipment). I don't know the design of your Beseler head, but it might be feasible to make a similar system for it.

jlbruyelle · Jun 3, 2017

~andi said:
I think a LED light-source can be dimmed without changing the color temperature.

The change is far less than with a halogen bulb, but there is definitely a change. The question is what amount of variation Chan Tran is willing to accept. For quantitative data and explanations, one can refer to https://www.digikey.com/en/articles/techzone/2014/feb/led-color-shift-under-pwm-dimming. And of course, there is also the question of the colour rendition index: a typical white LED is not very good under this repect, the very best ones achieve an almost perfect 98 but they are expensive. Again, it all depends on how close to a perfect colour source you want it to be.

rbrigham · Jun 3, 2017

Hi Chan

I work in the film industry

we dim incandescent and discharge lights without changing the colour temperature by using scrims mostly
they are metal wire disks that slip in behind the barn doors
they come in 1/2 and full stops and can be combined
we are also using LED lights more and more
these dim pretty well without changing colour temperature

ND filters work as well but generally do change the colour if you layer a few up

what lights do you need to dim ?

Sirius Glass · Jun 3, 2017

My dichroic head is voltage controlled so I would not lower the voltage to save electricity. If I need less light, I can adjust the iris or raise the enlarger. The latter would change the size of the image too.

Bill Burk · Jun 3, 2017

jlbruyelle and rbrigham have the right ideas, physically blocking a percentage of the light will dim it, without changing its color temperature.

Also since you mention using it to check light meters, I wanted to alert you to an idea:

Search eBay for Photo Research Reference Light Source, there are a couple up there now that are not too expensive.

I got one of the newer blue ones for not too much money and it "worked" as soon as I turned it on. 100 foot lamberts is a good brightness for the purpose, it's high on the low scale and low on the high scale of many meters, for instance the SEI Photometer and Weston Master.

~andi · Jun 4, 2017

jlbruyelle, thanks for the link to the PWD dimming article. Very interesting!

Chan Tran · Jun 4, 2017

Where is the link?
I found the link. I am afraid of using the PWM technique because I found that a a number of meters read erratically with a modulating light source.

darkroommike · Jun 4, 2017

Dichroic heads (other than Durst) do not create neutral density with equals amounts of all three filters. For really bright and hot light sources you could use Waterhouse stops of scrims as others have mentioned. For lower output sources, something like Lee or Rosco light modifiers are pretty good but almost all ND grays dyes have some color bias.

jlbruyelle · Jun 4, 2017

Chan Tran said:
Where is the link?
I found the link. I am afraid of using the PWM technique because I found that a a number of meters read erratically with a modulating light source.

Well, it would not be very difficult for an engineer to develop a continuous-current regulator for a LED, the main problem is that its efficiency would be very lowi and it would heat much, much more than a PWM regulator.

KenS · Jun 4, 2017

Try with a gelatin ND filter either above or below the lens.

Ken

MattKing · Jun 4, 2017

You could move the light source farther away.

pentaxuser · Jun 4, 2017

darkroommike said:
Dichroic heads (other than Durst) do not create neutral density with equals amounts of all three filters.

Good to know as I have a Durst but what's the reason why the Durst heads manage this but not other makes of enlargers?

Thanks

pentaxuser

Sirius Glass · Jun 4, 2017

My Omega enlarger lets me choose between a a high and a low light setting.

darkroommike · Jun 5, 2017

pentaxuser said:
Good to know as I have a Durst but what's the reason why the Durst heads manage this but not other makes of enlargers?

Thanks

pentaxuser

The Durst system was designed this way from the start and used a log-10 scale on all three colors, a match up for their analysers. Other makers chose to use Kodak or Agfa color printing equivalents for the scale on the dicrhoic filter knobby things.

ic-racer · Jun 6, 2017

Chan Tran said:
I need a light source to test camera exposure meter as well as testing camera shutter speeds on auto.
I used to use a Beseler computerized 45 dichoic head for this. I just dial in equal filters to create neutral density.
I am thinking of a few ways.
1. Copying the dichroic head design using a mechanism that moves the filter in and out of the light path but instead of using filter I would use just an opaque gate to reduce the amount of light.
2. Inserting ND filter in the light path.

Dimming the light by reducing power would lower the color temperature.
Any ideas?

If you are worried about diffraction, Durst used two sliding doors that opened and closed between the lamp and the diffuser box to control light intensity. Uses a knob on the head just like the three color filters.
If you are not worried about diffraction, how about projecting the light through a lens with a variable aperture.

Chan Tran · Jun 7, 2017

ic-racer said:
If you are worried about diffraction, Durst used two sliding doors that opened and closed between the lamp and the diffuser box to control light intensity. Uses a knob on the head just like the three color filters.
If you are not worried about diffraction, how about projecting the light through a lens with a variable aperture.

No I don't worry about diffraction as I don't actually take photo with it. I would like a light source that can change intensity over something like 6 stops range and not varying its color temperature. Like I said I am thinking of using a variable gate. I want to be able to adjust continuously rather than in steps. In steps is OK but it would be hard to make the steps exact.

ic-racer · Jun 11, 2017

Chan Tran said:
No I don't worry about diffraction as I don't actually take photo with it. I would like a light source that can change intensity over something like 6 stops range and not varying its color temperature. Like I said I am thinking of using a variable gate. I want to be able to adjust continuously rather than in steps. In steps is OK but it would be hard to make the steps exact.

I guess I'm not sure why not a lens with aperture? That is the universally common method of altering light intensity without changing color temp.

Sirius Glass · Jun 11, 2017

Sirius Glass said:
My dichroic head is voltage controlled so I would not lower the voltage to save electricity. If I need less light, I can adjust the iris or raise the enlarger. The latter would change the size of the image too.

ic-racer said:
I guess I'm not sure why not a lens with aperture? That is the universally common method of altering light intensity without changing color temp.

Great minds think in similar paths.

Chan Tran · Jun 12, 2017

A diaphragm is expensive and small. I don't want a lens as I want to light to diffuse. I think a sliding gate like them enlarger is fine.

Bill Burk · Jun 14, 2017

Even if it isn't easy to make a stepped gate precise (probably easier than you think though), a stepped gate would be more repeatable. And that might make it a better plan.

EG&G sensitometers have a deep well with a flash bulb at the bottom. The light modifiers you are supposed to use are like grates. There are a few of them and they simply have different total area "cut" out of an opaque sheet.

AgX · Jun 14, 2017

darkroommike said:
The Durst system was designed this way from the start and used a log-10 scale on all three colors, a match up for their analysers. Other makers chose to use Kodak or Agfa color printing equivalents for the scale on the dicrhoic filter knobby things.

All use log-10 scales, but with different factors. I know of 3 different scaling systems.

darkroommike · Jun 16, 2017

AgX said:
All use log-10 scales, but with different factors. I know of 3 different scaling systems.

On my Durst 30m-30m-30c is log base 10 of 2 aka ND 0.3 aka 1 f/stop. That's pretty nifty and Kodak CP10M+CP10Y+CP10C is not ND 0.3 nor is Beseler Dichro or Omega Dichro or Agfa Dichro

ic-racer · Jun 26, 2017

A little OT but Omega D5500 head is similar in scale to Durst: (there was a url link here which no longer exists)

How to dim a light without changing its color temperature

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