Andrea B. Posted February 5, 2016 Share Posted February 5, 2016 Experiment: Take a typical UV and IR foto and compare the channels to see where UV and IR light is being recorded by a typical converted digital camera.Rather broadly defined, yes. But it will be fun to look at. Result: In both UV and IR fotos, light is recorded in all three channels. Please comment and do point out errors!! I have no embarassment about screw-ups and am happy to make corrections. If I don't try, I don't learn. [Anyone embarassed about screw-ups should not be making experiments before the well-educated UVP readers!] Equipment: Nikon D600-broadband + Coastal Optics 60/4.0 + Sunlight REFERENCE FOTOS: As shot with my usual in-camera WB settings which are for convenience. No conversion, no profiling, just Jpg extraction shown in these Ref fotos. Visible Reference: Baader UV/IR-Cut Filterf/8 for 1/1000" @ ISO-100My in-camera convenience WB setting for Visible work is fairly accurate. Infrared Reference: B+W093 IR-Pass Filter (830nm)f/8 for 1/200" @ ISO-100The preceding Visible setting was used for the IR (which is monochrome). Ultraviolet Reference: BaaderU UV-Pass Filterf/8 for 1/15" @ ISO-100My in-camera convenience WB for UV work is used because it is as close the actual raw composite as I can push a Nikon in-camera white balance (at this time). RAW COMPOSITES:Here is what was really recorded as seen by Raw Digger after demosaic, gamma curve and end-point balance (which are required to make the raw suitable for human eyes). Minor alterations have occured due to resizing and stuffing into an sRGB Jpg box. Raw Composites look dull. NO white balance has been applied. Visible Raw Composite:There is 1% underexposure in the Red channel, .5% underexposure in the Green channels, and 3.3% underexposure in the Blue channel. Infrared Raw Composite:There is no under-exposure or over-exposure. Ultraviolet Raw Composite:There is 12.7% underexposure in the Blue channel and 1.3% underexposure in the Green channel. The Red channel has come close to overexposure. CHANNEL HISTOGRAMS:The x-axis shows EV in increments of 1/3. The y-axis count is linear. The overall range is based on the D600. Channels seem to oversaturate around EV+3. The histograms for the entire photo do not seem particularly interesting to me. After this I'll take some samples so we can better compare how UV and IR are recorded. Visible Channel Histogram: Infrared Channel Histogram:As per the way I have RawDigger set to Auto-analyze over- & under-exposure, this shot is neither. Still in the real world I think I could have let in a little more light. Ultraviolet Channel Histogram: SKY SAMPLES:Now, to me, things get interesting when you look at sampled portions of the photographs.I took a small squarish sample from the sky in the upper left corner so I could compare the UV and IR recordings.In UV light, skies go bright. In IR light, skies go dark.So we should expect to see this in the sample histograms. The sky samples are shown as a raw composite with no white balance applied. Visible Sky & HistogramThe Visible sky is recorded as raw cyan with blue leading just a bit.The EV between 0 and 1 is between the IR and UV values which follow. Infrared Sky & HistogramThe IR sky is recorded as basically monochrome and darkish around EV -3. Red and blue lead just a bit over raw green. Ultraviolet Sky & HistogramThe UV sky is recorded as bright orange-red around EV +2. The large amounts of blue & green desaturate the raw red. Green leads blue to give the orange push. SHRUB SAMPLES:This comparative sample is taken from the evergreen Euonymous shrub.In a manner opposite to the sky, in UV leaves go dark. In IR, leaves go light.Again, we should see this in the sample histograms. The sky samples are shown as a raw composite with no white balance applied. Visible Shrub & HistogramThe Visible leaves are dark brownish-green around EV -2. Infrared Shrub & HistogramThe IR leaves are monochrome and moderately bright in winter light at EV +1. Ultraviolet Shrub & HistogramThe UV leaves are very dark red around EV -2.5. Link to comment
Andrea B. Posted February 6, 2016 Author Share Posted February 6, 2016 So what would the three original fotos look like if converted & edited?Just test scenes, so nothing much to write home about. :) There is a woodpecker in this foto. This one too. Unresized excerpts 2/3 of which contain a Downy Woodpecker.Vis/IR/UV Link to comment
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