ABSTRACT This research project aims to investigate and illustrate the effects a light source s spectral distribution and colour temperature has on photographic image colour reproduction, and how this often does not correlate with how the human eye processes the same input. Under varying lighting conditions, a grey card kit was photographed several times with the camera s white balance set to both Auto and 5500 Kelvin. The investigation will show that blindly relying on a camera s auto white balance software often can result in a photographic quality less than optimum. Being familiar with these effects are essential to any photographer that is going record a scene with several light sources of different colour temperature. INTRODUCTION METHOD For all figures, a grey card kit including a grey scale card, a grey scale card and a colour card was fixed at an angle approximately 45 between the imaging capture device and the light source. A lot of effort was put into ensuring that only a single type of light source was used for each photo (however, interference from other types of light sources was at times impossible to avoid). This was done to illustrate the effect said light source had on the grey card kit as well as the surroundings. The camera speed (ISO) was set to 100 (the slowest available), to decrease the amount of noise interference.
RESULTS
Figure 1. Direct sunlight early morning 0-1 hr after sunrise Decent amount of contrast, tones were warm. POOR > 1 2 3 4 5 6 7 8 ( 9 ) 10 < PERFECT Due to the angle of illumination during sunrise and sunset, the light transmitted from the sun travels through a thicker layer of atmosphere, causing absorption and scattering of sunlight. Such scattering mainly affects light of shorter wavelengths, causing the light that actually is transmitted to be of longer wavelengths, hence giving the light we see a more reddish appearance. During daylight, the human photopic eye is more sensitive towards the longer wavelengths of the spectrum, making us see sunrises in a warmer tone.
Figure 2. Direct sunlight Midday Fairly bright. Neutral colours. POOR > 1 2 3 4 5 6 7 ( 8 ) 9 10 < PERFECT Near noon, the combination of light from the sun, sky and clouds usually has a colour temperature in the region of 6000 to 6500 K. An overcast (cloudy) sky has a somewhat higher colour temperature, while that of a blue sky may become as high as 12 000 to 18 000 K. [1] The reason that the scene seems slightly colder is because of the heightened colour temperature that gives of a more bluish light.
Figure 3. Skylight (open shade with clear blue sky on a sunny day) Slightly less contrast, slightly de-satured. POOR > 1 2 3 4 5 6 ( 7 ) 8 9 10 < PERFECT The colour temperature of the light from the sky and the clouds is of interest independently to that of sunlight, because it is skylight alone which illuminates shadows and gives them a colour balance that differs from that of a sunlit area. [1]
Figure 4. Sunlight reflected (bounced) from a building Contrast was good, due to the white wall the scene appeared very bright. POOR > 1 2 3 4 5 6 7 8 ( 9 ) 10 < PERFECT The sunlight was reflected off a non-coloured metallic surface that did not induce any coloured polarized light, leaving the entire spectrum still visible. This resulted in a white light, both to the human eye and a digital capture sensor.
Figure 5. Tungsten light (3200K) Very warm tones, slight de-saturation. Predominant in red / yellow. POOR > 1 2 3 4 5 ( 6 ) 7 8 9 10 < PERFECT Tungsten filament lamps has a melting point of 3650 K. The tungsten lamps used for domestic purposes places with a colour temperature between 2760 and 2960 K. This colour temperature appears very red, both to the eye and a digital sensor.
Figure 6. Candle light Even warmer colour tone than tungsten. Very dominant in red / yellow colours. POOR > 1 2 3 ( 4 ) 5 6 7 8 9 10 < PERFECT The colour temperature of a candle light is even lower than that of a tungsten lamp, only 1930 K, resulting in an even warmer / more reddish appearance.
Figure 7. Fluorescent light Decent contrast, somewhat de-saturated. POOR > 1 2 3 4 5 6 7 ( 8 ) 9 10 < PERFECT Fluorescent lamps are low-pressure mercury-vapour discharge lamps emitting visible light, which has been converted from short-wave UV radiation. This results in a light quality similar to that of continuous-spectrum lighting, attempting to appear somewhat like daylight.
Figure 8. Pink street light (high pressure sodium) Strong contrast, very yellow-ish (with a tint of pink) tones. POOR > ( 1 ) 2 3 4 5 6 7 8 9 10 < PERFECT One of the things that separates High Pressure Sodium lamps from LPS (Low Pressure) is that they contain mercury. This addition, as well as a broadening in the spectrum emitted, makes it easier to distinguish colours in objects illuminated while still maintaining low operating costs.
Figure 9. Soft yellow light (low pressure sodium) High in contrast, strong yellow colour. POOR > ( 1 ) 2 3 4 5 6 7 8 9 10 < PERFECT The light emitted from a LPS is a virtually monochromatic yellow light, averaging at a wavelength of 589.3 nm. This narrow bandwidth yellow light makes it very hard to distinguish colours of any illuminated object. However, the output wavelength of this light is very close to the peak sensitivity of the human eye, making them ideal for both street lighting and security lighting.
Figure 10. Blue-white street light (mercury vapour) Good contrast, appeared bright in white and with a tint of blue. POOR > 1 2 3 4 5 6 7 ( 8 ) 9 10 < PERFECT Mercury vapour lamps attempts to imitate the qualities of sunlight with a colour temperature of 5500 Kelvin. This is however only achieved to a certain degree, seeing as the light is still distinctly bluish in colour.
DISCUSSION REFERENCE LIST Photographic light sources in Attridge, G.G., Axford, N.R., Jacobson, R.E. & Ray, S.F 2000, The Manual of Photography: Photographic and Digital Imaging, 9 th edition, Focal Press, pp. 16-38 [1] Photographic light sources in Attridge, G.G., Axford, N.R., Jacobson, R.E. & Ray, S.F 2000, The Manual of Photography: Photographic and Digital Imaging, 9 th edition, Focal Press, pp. 25 http://www.absoluteastronomy.com/topics/sodium_vapor_lamp - Regarding Sodium Vapour lamps, accessed 18 th april http://www.lamptech.co.uk/documents/so1 Introduction.htm Regarding Sodium Vapour lamps, accessed 18 th april APPENDICES