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6 - Reflection & Transmission

This study delves into spectral reflectance curves illuminated by various light sources, such as low- and high-pressure sodium lights, incandescent bulbs, and coated incandescent lamps. It discusses how these sources render color differently, along with phenomena like color-coding and "false color" representation. The effects of filters, including their combined transmissions in series, are analyzed through mathematical laws governing light transmission and absorption. This work highlights modern digital imaging techniques while considering future developments in photon detection and color representation.

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6 - Reflection & Transmission

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  1. 6 - Reflection & Transmission

  2. Spectral Reflectance Curves

  3. My hand illuminated by different light sources Low Pressure sodium - two monochromatic lines near 590 nm - very “orange” High pressure sodium - broader SPD - “pinkish” Incandescent - very broad SPD, but tends to look reddish compared to sunlight Coated Incandescent - coating makes it appear more like sunlight

  4. Color-coding & “False Color” • Normally represent green with green, etc. • Sometimes color shifts occur if not faithfully reproduced - green versus greenish • Sometimes gross errors occur in coding - green original versus non-green representation • Can also deliberately re-code: especially useful in representing light the human eye cannot see

  5. Landsat 7 scene was imaged in 4 different wavelengths: Band 1 Blue 470 nm Band 2 Green 555 nm Band 3 Red 660 nm Band 4 IR 880 nm

  6. Transmission Through Filters (more) Most colored filters - dyes (organic & inorganic) “Color-blind” filters: Black or gray absorbers - photo B&W negatives Scatterers - ground glass

  7. Filters in Series T1 T2 T3 T4 Two filters with transparencies T1 and T2 combine so that the net transmission T12 is just: T12 = T1T2 If we had 3 filters, the result would be T123 = T1T2T3 If we had N filters, we would getT123…N = T1T2T3…TN If all the filters in our series were identical, this simplifies to: TN = TN I0 I1 I2 I3 I4 I1=I0T1 I2=I1T2=I0T1T2 I3=I2T3=I0T1T2T3 I4=I3T4=I0T1T2T3T4 So T4=I0T1234 where T1234=T1T2T3T4

  8. Example, 8 identical filters, each with T= 1/2. The net transmission would be: • T = (1/2)8 = (1/28)  0.0039  0.4% • I0 I • I = I0e-ax = I0e- • a = absorption coefficient per unit thickness the material • x = thickness • = ax the “optical depth” e = 2.7182….. Effect of increasing thickness -(Bouguer’s Law) Effect of increasing absorption coefficient (Beer’s Law)

  9. Smooth and Rough Surfaces

  10. Driving on a wet road at night………. Hard to see your own headlights on the road - can’t see painted lines! Glare from headlights of oncoming cars reflecting off the water on the road Very bad if angle of incidence > Brewster’s

  11. Color Effects The net transmission through a series of filters must be calculated at every wavelength: T12,440nm = T1,440nmT2,440nm T12,550nm = T1,55nmT2,550nm etc. or in general, T = T1,T2, Similarly, the reflection at a surface must be done in the same way

  12. Older color cameras used dichroic filters

  13. Modern digital cameras use charge-coupled devices CCDs These are intrinsically B&W detectors, but can be “coded” to make color images

  14. The Future: Digital devices that “tag” the photon with its energy, hence wavelength - “3D” detectors!

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