Tim Parkin - Landscape Photographer

Ever since I started using my v750 scanner I had read about the use of different types of profiling target and there has always been something that I couldn’t quite understand. Why do we need different profile for different film types?

Originally when I bought my scanner, it came with a Monaco EZColor target which (I beleive) is on a Velvia transparency. Now I originally presumed that the transparency had a bunch of colour targets which were measured accurately and that when I scanned them in I would get a set of ‘adjustments’ to compensate for any colour shifts in my scanner sensor.

However, I then started to hear about different targets for different films and started to doubt my understanding of how the profile targets worked. Why would we need different film bases if the colours of each patch was measured from the actual transparency itself? Surely a standard blue measured from Velvia would be the same as a standard blue measured off Provia. Just to reiterate, profiling targets as I understood them were exposed in order to create the same final colour on the transparency – they are not photos taken of standard colour targets. However, I started to wonder if they were equivalent to photos of gretag macbeth targets.. Why else would you need different ones for different films.

The clue was in the types of target available though. Wolf Faust, a manufacturer of affordable targets, sells a Velvia target and also a target that works for both Astia and Velvia 100. Well obviously Astia and Velvia 100 are very different in colour palette so there must be something else going on.

After some digging around, I found a couple of articles on metamerism in transparency film. Now metamerism is something that you may have heard of in relation to inkjet prints. Some older print colours change colour value under different types of light. For instance, A flourescent light, although it may look ‘white’ is actually has very sharp spikes in it’s frequency curve. If you look at the picture to the right, you can see the flourescent has big spikes in the blue and
another big spike in the yellow. This big spike, if it matches some of the chemicals in the film dyes, can cause those dyes to emit more light (or absorb more light). Hence you may see a boost in a certain colour component under flourescent light. This was the clue I needed, and with a little further digging I found confirmation (albeit scattered around the web).

So, hopefully I can describe it to you.. The key thing to understand is that the dyes used in film display some limited form of metamerism, as described above; They shift colour slightly under different types of lighting.

So for different types of scanner light source you may find that the dyes in your film cause a colour shift. So, LED or flourescent lights may cause the red dyes in Velvia type film to emit magenta light(1) whereas a well calibrated light source would give more accurate colours.

So now we can understand why one profile may work for Astia and Velvia 100. Although these two films produce different colour palettes, the dyes used in the films are fundamentally the same(2).

This is probably why the making of colour profiles is so difficult. You need to have perfect light sources and also a very accurate specrometer in order to get ‘baseline’ readings from film targets.

I had a fairly long email and phone conversation with Ian Scovell (who operates probably the best set up and managed Imacon in the country!) and we agreed that this sounds about the best explanation we can come up with for the differences.

I hope this helps and if anybody has any experiences with scanning targets that can add to the discussion, please add a comment below!

(1) This is just a guess by the way – it may not be the direct cause of magenta colour in scans of velvia although it sounds plausible.

(2) The different colours are probably due to different levels of couplers

UPDATE: My talk about emitting and excitation, while theoretically possible, is most likeley only a minimal cause. On further thinking about the problem, colour changes in detected light for different dyes can be simply explained by the interaction of the light source spectrum with the dye absorption spectrum. I’ll try to find more info about this and add further notes..