Digital Orthochromatic Photography

I’m playing with ways of emulating early orthochromatic film, if anyone knows any technical aspects to this that might help – feel free to suggest ideas. So far I’m just winging it.

Light Response of Film
In terms of light covered it goes: Blue-sensitive, orthochromatic, isochromatic, panchromatic (all visible wavelengths) then super-panchromatic.

On the right is a photo of British explorers taken with orthocromatic film. Note the red on the flag is much darker than the blue area because the film isn’t receptive of red light. Looking at the spectral sensitivity (here) and (here) it’s clear the violet/blue area is the predominant band with it extending to green/yellow as well.

Test Ideas
I took an image from Google, something with the British flag, and made different Channel Maps for it. The greyscaled versions are the output images.

I used the QSE tool on WavelengthPro to make a 400nm image (the violet/blue peak) and a 580nm (the green/yellow peak) and made a 2to3 map of the peaks, the colour response worked well but the data-loss from interpolation was too much. Using the original [R,G,B] channes means I won’t lose any detail, so I tried using mainly those and came up with two maps that were pretty good – GBB and GBBV. Below are those maps and the RGB map for comparison.

This is the RGB (panchromatic) version, the red cross on the flag is lighter than the blue parts and the sky is dark.	This is a GBBtoRGB version, the cross appears darker. It’s not really true to the actual spectral response of the film.	This is a GBBVtoRGB version, the V is the interpolated deep violet image. Getting there…

Almost UV Photography

For ages I have wanted to do full-spectrum photography, which captures light from Infrared (IR) all the way to ultraviolet (UV), but the UV aspect of it is bloody expensive! DSLR sensors, both CCD and CMOS, capture light slightly outside the visible spectrum (VIS) but use things like hot mirrors and UV filters to narrow the band closer to 390-700nm. The sensors use channeling methods like a Bayer filter to give us the very useful RGB channels, in this post we will work with extra channels for IR and UV.

I am always looking for cheap alternatives for UV and I thought I’d test out a bit of a long shot – using a UV filter to maths my way to a UV image. To do this I bought a daylight simulating bulb that emits UVA (400-315nm) and some flowers from the local gas station. It’s a simple idea, the extra light that the UV filter blocks must be UV light so if we subtract all the other light we are left with UV.

No Filter – UV Filter = UV ResidueI subtracted each colour separately for each pixel: [r₁-r₂, g₁-g₂, b₁-b₂], it was rather red so I used the red channel for the new R,G and B making a brighter grayscaled image (see below). Then I used that new “UV” image along with the colour image to map channels [GBU to RGB] like the images Infrachrome makes using this technique. For infrared and ultraviolet he uses an adapted camera specifically for full-spectrum, infact he uses two in a fantastical and magical set up. Unfortunately mine didn’t work very well, my first guess was that the lower range of blue light being reflected as there is no sign of a nectar guide. But after consulting a pro UV photographer I was told it is due to infrared-leakage.

I thought I’d do a full spectrum map whilst I had the camera set up so I put on a 950nm IR pass filter and took another shot. In the above image the far right is the channel map of the other three.