Dichroic enlarger ? Cold-light source ? Please help me !!

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Hello !

I know what a B&W condenser head looks like and how to use it ... I know what a B&W diffuser head looks like and how to use it ...

But ... I have read a few about DICHROIC head, and I have a hard time trying to find information on it. The same for cold-light source.

What are they used for ? B&W ? Color ?

Please help me !

Thanks a lot for any details !!!!


-- Anonymous, March 26, 1997


Dichroic enlarger/Cold light head

The term "dichroic" refers to the type of filters that is used between the light source and the negative; they are interference filters (as opposed to coloured glass or gelatin filters), where the transmitted colour and the reflected colour are opposite. E.g., a yellow interference filter shows a blue reflection, hence the term dichroic, "two-colored". They are used in colour heads (Yellow, Magenta, and Cyan filters) and for b/w variable contrast heads (Yellow and Magenta filters), where you change the paper contrast with the light colour. Diffuser types with a "mixing chamber" and a piece of white acrylic above the negative are most common with these heads, although some manufacturers do use additional condensers. Light source is usually a halogen lamp, often with an additional reflector. The latter type is sometimes called cold mirror lamp (because it only reflects visible light and transmits the infrared) but should not be confused with a real cold light head!

A cold light head is for b/w only and the light source is a fluorescent tube. The fluorescent tube does not heat up the negative very much (very little infrared), hence "cold" light. In comparison to the other heads, the spectrum contains a large amount of blue, resulting in comparatively short exposure times with graded b/w paper. Cold light heads are much more common in the US than in Europe. The light characteristic is also diffuse, the light from the tube(s) is scattered through one or two sheets of white plastic above the negative. Since a few years, models with two differently colored tubes are available for use with variable contrast papers. Hope this helps,

Arne Croell

-- Anonymous, March 26, 1997

Let's see if I understand ....

Thanks a lot for your answer, it helps !!!

The following are not quite affirmations ... they are rather supposition ... please tell me if they are correct :

So, in color enlargers, you have three filters, a Yellow one, a Magenta one, and a Cyan one. In order to enlarge a color negative, you have to the three of them. But you also can use such an enlarger to make B&W prints, by only using the Yellow filter and the Magenta one ...

Is that OK ?

And if so, what is the very difference between a "color" head and a "dichroic" one ??? Only the kind of filters they use ??? Are they ised for the same purpose ? Is one considered better ?

Thanks for your help !

-- Anonymous, March 26, 1997

Dichroic Head

Yes, a dichroic head can easily be used for b/w and b/w varariable contrast paper. It might be an expensive option if you only want to do b/w.

In most cases, color head and dichroic head are synonymous. There are a few colour heads around working on different principles, e.g. those using three light sources (blue green red additive filtration) which might not use interference filters, as opposed to the common ones with the subtractive interference filters and one light source. One advantage of interference filters as compared to others (gelatin filters in a filter drawer) is that they cannot bleach out, as the colouring is due to interference effects within thin layers on the glass support (the same as an antireflection coating on a lens) and not due to absorption.

-- Anonymous, March 27, 1997

New Cold Light-Aristo

In the discussions about printing B&W -Variable Contrast it should be noted that Aristo has produced a new cold light bulb that is faster than their previous product and is balanced to use conrtrast filters 'normally'. That is, without the yellow filtration that was needed for VC printing in the past. You can buy this bulb Model V-54 as a replacement or in a new head. for me it has made a great difference in my printing speed and results.

-- Anonymous, March 28, 1997

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