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Blacklight & Digital Cameras

Published : 9th December 2004

As many meters do not respond to blacklight as they are not sensitive enough to the UV spectrum, would it be safe to say that one might use a digital stills camera to gauge final exposure (with the addition of the camera filter over the lens)?

The ASC manual mentions several Wratten filters for use in blacklight photography, shielding the lens from BL light going to the subject allowing only light emitting from the luminescent material. These are specified as Wratten 2b,
2E and 3.

I am using a wildfire fixture, is there a particular filter that one has found to be better than others? Are better results obtained with a tungsten or daylight stock.

Nick Paton
Director of Photography
Brisbane, Australia
www.npdop.com


>The ASC manual mentions several Wratten filters for use in blacklight >photography, shielding the lens from BL light going to the subject >allowing only light emitting from the luminescent material.

Bear in mind that if you properly filter the UV out of the lens (and I have used a 2B for this in the past) you are now only photographing visible light, and your light meter should be no more inaccurate than usual.

Personally, I would be as nervous about using a digi-still camera as I would a light meter, since there is nothing to reassure me that the digital still camera's frequency response (or spectral sensitivity curve) matches the film stock I am shooting. If you shoot some digi stills and some film stills and compare them, you should be able to use the digi-still camera as an exposure gauge, but since you are filtering the UV out of the lens anyway, you are back in the visible light world. I would suggest metering with a spot meter through a UV filter to make sure that the UV bouncing around wasn't biasing your readings.

I have only shot UV-activated pigments with tungsten balanced film myself, so I have no direct advice on tungsten vs daylight film - but if you think of the UV light source as the "electrical power" that powers the fluorescent pigments - what you are photographing is visible light emanating from those pigments.

The differences between a 2A 2B. 2C and 2E (which all appear a little bit yellow) have to do with exactly where they cut off - if you do not have violet coloured items in the frame, they will all act very similar to each other. I don't have the filtration curves handy for the 3, but the others range from 385 nm for the 2c to 390nm for the 2B to 405 for the 2A to 415 for the 2E.

The bigger the number (the longer the wavelength) the less violet light will be transmitted.

Unlike the 0 and 1A filters, these filters all cut off pretty steeply.

I hope this is somewhat useful.

Mark Weingartner
LA based


Nick Paton wrote :

>I am using a wildfire (UV) fixture, is there a particular filter that one has >found to be better than others?

Mark Weingartner covers the topic very well. I would like to add only a couple of things.

Subject matter, and how you want to render it has a prime place in planning. I am assuming you are shooting in a darkened environment, where the only luminance comes from the black lights. If you are shooting artificial colors, with no discernible relationship to reality (compared to, for example, rendering flesh tones), as you would be if shooting fluorescing rocks or pigmented paints, then you would want to alter your approach according to that which will provide the most desired coloration. For instance, if your fluorescing coloration predominantly tended toward one part of the spectrum more than another ( warmer reds and oranges compared to cooler blues and greens) you might choose daylight film, since tungsten film would, if uncorrected as with an 85, appear generally cooler to begin with for a given color temperature of light. For that reason, you might use tungsten film without an 85 for rendering stronger blues. Since most digital stills cameras have both tungsten and daylight settings, you might test these to gauge the differences.

In a darkened environment with only blacklight for illumination, you will need a generally longer than usual exposure. Bear in mind that distance to the subject is important, since the effect of light intensity reduction over distance is greater than usual in this type of imaging. Prior testing is critical, and Mark s comment about correlating your digital stills camera tests with your actual film test results is paramount.

One more thing- according to the Kodak book, the Wratten #3 cuts off at about 430 nm.

Many of us would be interested to hear how things turned out, if you'd be so inclined...

Ira Tiffen
LA, CA


Nick,

I am assuming you are trying to photograph the fluorescence of certain paint oxides or "glowing" subject materials in total darkness in posing this question. As you have discovered, an incident meter is not useful here, and that was certainly the case in my testing a couple of years back.

There should be a thread in the CML archives on this.

My rule of thumb is to use a reflective reading of the mid tone fluorescence area to obtain a mid grey exposure, then open up two stops on the lens. I found daylight stock to react the best to black light, for aesthetic reasons...the tungsten seemed a little muddy, and I used a Haze 2 filter to protect the film from the bombardment of UV light which obtained a relatively sharper image. From memory I used a 250 ASA
daylight stock.

And one more thing, the closer the black light source to the axis of the camera , the more reflectance the camera can read.

This method worked for my eyes, let me know how you get on.

If testing with a digital stills camera, I'm curious to see if the same techniques apply.

Kodak have a number of extensively detailed technical papers on fluorescence photography.

Good luck, and test, test, test.

Cheers

Simon Higgins
DP
Australia


Ira Tiffen writes :

>I am assuming you are shooting in a darkened environment, where the >only luminance comes from the black lights. If you are shooting artificial >colors, with no discernible relationship to reality

We are thinking about shooting a wall lit by wildfire fixtures and shaded with their paint The wall will be lit with blacklight while the foreground will be tungsten light (none spilling onto the wall).

Here is my approach as I understand it :

Light rear wall with wildfire paint with a wildfire flood fixture. Camera filter is one of the Wratten filters as described previously. Use the camera filter over the spot meter to ascertain visible light reflected from paint on b/g, exposure set to spot meter as per manufacturers film stock rating. Foreground : Shoot MacBeth with uv filter on camera, Minolta 4F ASA set to film stock rating adjusted accordingly to the filter factor correction of a UV filter.

By cutting out the bottom of the visual light spectrum by using the UV filter will I get natural skin tones on my tungsten lit f/g?

Nick Paton
Director of Photography
Brisbane, Australia


>My rule of thumb is to use a reflective reading of the mid tone >fluorescence area to obtain a mid grey exposure, then open up two >stops on the lens.

I am assuming when you say you opened up by a couple of stops you weren't metering through the filter?

Nick Paton
Director of Photography
Brisbane, Australia


>The wall will be lit with blacklight while the foreground will be tungsten >light (none spilling onto the wall).

I think you are on the money in terms of :

1/. finding the exposure of the "UV activated fluorescent pigment" back wall
2/. lighting the "tungsten lit" foreground to balance
3/. Shooting a MacBeth [and a gray scale] to aid in timing out the color bias of the filter

I don't think any of the discussed UV-blocking filters reduce visible light enough to affect visible light exposure- the reason to meter (and shoot) through one of them is to stop stray UV from affecting your readings (or adding exposure to your film in the UV end of things)

I do not believe any of these filters will ruin your ability to record good skin-tones, but they are slightly yellow tinged, so the MacBeth and gray scale (or grey scale, I suppose) will help get rid of the yellow in post.

If your BK effect-painted wall is big, you might consider washing it with two UV lamps - one from each side - since more UV will give you more visible light back from the fluorescing pigments...and, more importantly, an even wash will give you even response from your pigments.

Please let us know how the Digi-still camera records the scene - since you are blocking out the UV light and recording only the visible light, it should be exactly as useful as a guide as it is for "normal" work - if you get a chance, take a photo with and without the UV filter on the still camera and let us know how the two images differed.

You should be able to trust your eye in balancing the foreground to the background - the only thing that you might find tricky is that some of the fluorescent pigments are very "narrow band"...that is to say very saturated monochromatic colors, and as such, a spotmeter might under-report or over-report them compared to how they photograph. Your digital still camera will most likely be more useful in judging those specific colors.- or at least will help provide a little "peace of mind" that you are in the ballpark.

Mark Weingartner
LA based


>Please let us know how the Digi-still camera records the scene

Will do, thanks for your help.

Nick Paton
Director of Photography
Brisbane, Australia


This is an update of information ...published 16th Jan. 2005...

Some of you may remember me posting some months back looking for appropriate filtration to exclude blacklight wavelengths to the camera while capturing the visible paint output from wildfire materials. Ira Tiffen was on the money with the use of an LLD. Follow the link for an example.

Foreground was shot with coloured tungsten and the background was shot with a wildfire flood. The paint was applied to two black flats using an overhead projector and pattern to give a forced perspective feel. The tungsten instrument was a 15ow dedo.

I found exposure for the paint (through the filter with a spot meter)and then lit the talent plus 1/3 incident to allow for the stop loss through the filter :

http://members.optusnet.com.au/nicholaspaton/

Nick Paton
Director of Photography
High Def./Standard Def./Film
Aaton Xtr Prod owner operator
Brisbane, Australia
www.npdop.com


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