It all boils down to the concept of color temperature. Color temperature is a way of measuring the quality of a light source. It is based on the ratio of the amount of blue light to the amount of red light, and the green light is ignored. The unit for measuring this ratio is in degree Kelvin (K). A light with higher color temperature (i.e., larger Kelvin value) has "more" blue lights than a light with lower color temperature (i.e., smaller Kelvin value). Thus, a cooler (resp., warmer) light has a higher (resp., lower) color temperature.
The following table shows the color temperature of some light sources
| Light Sources | Color Temperature in K |
| Clear Blue Sky | 10,000 to 15,000 |
| Overcast Sky | 6,000 to 8,000 |
| Noon Sun and Clear Sky | 6,500 |
| Sunlight Average | 5,400 to 6,000 |
| Electronic Flash | 5,400 to 6,000 |
| Household Lighting | 2,500 to 3,000 |
| 200-watt Bulb | 2,980 |
| 100-watt Bulb | 2,900 |
| 75-watt Bulb | 2,820 |
| 60-watt Bulb | 2,800 |
| 40-watt Bulb | 2,650 |
| Candle Flame | 1,200 to 1,500 |
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