Color Temperature Measurement and its affection.

Panasonic Corporation PAVBU Seigo Asada

Correlated Color Temperature • Color temperature is defined on the line which called the “Blackbody locus”. But not all existing light sources are on this blackbody locus. In fact ,most light sources having a white chromaticity point are slightly off of it. In such cases, the color temperature can be obtained using the isotemperature lines drawn across the blackbody locus. The color temperature is then referred to as “correlated color temperature. Blackbody locus (Color temperature is defined on this line.)

Correlated color temperature Is on the isotemperature lines.

Blackbody Locus on the uv-chromaticity diagram

Refer to http://www.konicaminolta.com/instrum ents/knowledge/index.html Color Temperature Instrument 1. Industrial measuring Instrument (Spectroradiometer and so on): 1. Measurement result is very precise. 2. It has a sensor which closely matches the sensitivity of human eyes. 3. It has spectroradiometer and analyzes precise spectrum distribution. 2. Colorimeter: Designed for film shooting 1. the result is different from industrial measuring instrument. 2. The sensors of photographic colorimeter is similar to the sensitivity of color photographic film. 3. Video Camera/ Digital Still Camera: Designed for video shooting. 1. the result is different from industrial measuring instrument and colorimeter. 2. It has a sensor which doesn’t match the sensitivity of human eyes because it is designed for video shooting. 4. How to measure color temperature at video camera. 1. RGB ration is different if color temperature is different as next page shows. Color temperature is calculated by inverse operation. However, the further light source is from blackbody locus, the more measurement error increases. Actual light source is not on the blackbody locus. So video camera’s color temperature display is not precise.

RGB Ratio from Planck's Law

RGB Ratio

2.500

Preset 2.000

1.500 R

Preset G Ratio 1.000 B

0.500

0.000 3000 3200 4300 4800 5000 5600 5800 6000 6500 9300 Color Temperature

Bulb R=G B=G Monitor Consumer TV in Japan Tungsten Lamp Cool White/Daylight Cloudy compact florescent light bulb Data spread of Color temperature • Difference by Sensor – Spectroradiometer: It meets human eye characteristics. – Colorimeter: It meets film characteristics. – Video camera / Digital Camera: It meets video characteristics. – Color temperature measurement by Video camera has some difference because the sensor is not for measurement. – Even if you use colorimeter, it has data spread. Every measurement shows the different value. Data spread is around 100K or 200K when light source is 3200K. • Difference by Lens – Lens has different characteristics and aged deterioration. • Company A: 2800K (Lens A) , 2900K (Lens B) • Company B’s No.1: 3000K (Lens A), 3000K (Lens B) • Company B’s No.2: 2800K (Lens A) , 3000K (Lens B) • Difference by Display Interval of color temperature. – Company A: 3000K,3100K,3200K,3300K ---- every 100K – Company B: 3000K,3200K,3400K,3600K ---- every 200K – Even if data error has 100 between A and B, display shows 200 error because display interval is 200K.

Data spread is more than 200 at best. Even the same company has 400 error by different model. What’s the difference between Industrial instrument (for ex. Spectroradiometer ) and Photographic colorimeter

• Spectroradiometer: In principle, near to the true value. • Photographic colorimeter: The purpose is determined appropriate filter for taking pictures. • You can see the big difference depending on light source the following graphs.

Refer to Konica Minolta homepage. http://www.konicaminolta.com/instruments/knowledge/index.html Conclusion

• Color temperature’s data spread is around 400K by model under Tungsten light. – Note: You have to use the identical lens when you compare camera A with camera B. Because if the lenses are different, color temperature will change. Appendix The difference between Color temperature change and human feeling about color temperature change

• You can see the difference of color temperature interval in case of low temperature and high temperature. 1000K change is big difference at 2000K. But 1000K change is small difference at 9000K.

Refer to http://en.wikipedia.org/wiki/Color_temperature Displayed by Mired (Inverse of Color Temperature)

• Relationship between inverse of color temperature and color becomes the same interval. • Definition of Mired = 106/T (T=kelvin temperature) – 3000K: 333mired, 6500k: 154mired, 9300K: 108mired – The difference between 3200K and 3400K (200K) is the same as the difference between 6500K and 7300K( around 800K). At 9300K, it becomes around 1500K. At 12000K, it becomes around 2400K. At high temperature, 1000K difference is not so big difference.

Color Temperature Conversion Filter

• Human feeling is in reverse proportion to color temperature. – Color Conversion Unit: • Mired = 106/T (T: Color Temperature) (Mired：micro reciprocal degree) – If original temperature is K1 and filter out temperature is K2, and then

– Reduce Color Increase Color Temperature Temperature Amber Filter Blue Filter • Each company has each model number – Fuji Film Model Number • Amber LBA1,2,4,,8,12,16

• Blue LBB1,2,4,8,12,16 – KODAK • +13~+5:81, +22~+14:81A, +30~+23:81B • -6~-15:82, -16~-26:82A -27~-38:82B • Next page shows model number comparison. Filter Model Number Comparison mired FUJI Film KENKO HOYA mired KODAK +135~+126 85B +116~+110 85 +85~+76 85C +160mired LBA16 LA160 +56~+47 81EF +120mired LBA12 W12 LA120 +46~+39 81D +80mired LBA8 LA80 +38~+31 81C +40mired LBA4 W4 LA40 +30~+23 81B +20mired LBA2 W2 LA20 +22~+14 81A +10mired LBA1 LA10 +13~+5 81 -10mired LBB1 LB10 -6~-15 82 -20mired LBB2 C2 LB20 -16~-26 82A -40mired LBB4 C4 LB40 -27~-38 82B -80mired LBB8 C8 LB80 -39~-50 82C -120mired LBB12 C12 LB120 -51~-61 80D -160mired LBB16 LB160 -80~-86 80C -108~-117 80B -127~-136 80A How to shoot by a film camera • Select a film type. – Daylight:5500K, Tungsten:3200K • Measure color temperature by color meter, select appropriate filter (Change display and then it shows filter number.) and then put it on a lens. Press Display button and then you can see 3 kinds of measured values. Measure color temperature LB index and change display LB index or (mired) LB filter. You can get a filter number. For example, if color meter LB Filter Number shows 6370K, LB index (kodak) becomes 25mired because Daylight film is 5500K. Color Temperature So you can use LBB1+2 (@Fuji for film. Film) or 81B (@KODAK). Mired must be used for color temperature difference.

Difference (Mired) 21 21 21 21 21 21 Original 3200 4300 5600 6500 9300 12000 Converted 2999 3944 5011 5719 7780 9585 Difference (Mired) -40 -40 -40 -40 -40 -40 Original 3000 4300 5600 6500 9300 12000 Converted 3409 5193 7216 8784 14809 23077 Difference (Mired) 80 80 80 80 80 80 Original 3200 4300 5600 6500 9300 12000 Converted 2548 3199 3867 4276 5333 6122 Difference (Mired) 120 120 120 120 120 120 Original 3200 4300 5600 6500 9300 12000 Converted 2312 2836 3349 3652 4395 4918

The same color temperature difference

3200K vs. 3000K (200K difference) 3000K vs. 3400K (400K difference) 6500K vs. 5700K (800K difference) 6500K vs. 8800K (2300K difference) 9300K vs. 7800K (1500K difference) 9300K vs. 14800K (5500K difference) 12000K vs. 9600K (2400K difference) 12000K vs. 23000K (11000K difference) Is necessary color temperature measurement in case of video shooting? • Film camera: Color temperature measurement is necessary because a film camera doesn’t have color temperature adjustment. You have to change film (Daylight or another films) or attach the color transformation filter on the lens to change color temperature. • Video camera: Color temperature measurement is not necessary because a video camera has white balance compensation system. White balance converts the input signal to the reference signal. After processing white balance, the signal doesn’t depend on color temperature. You don’ need to know color temperature. • That means even if display of color temperature has an error, the video camera’s color doesn’t change. – Note: White balance is so important. If white balance is not correct, color reproduction is not correct. Summarization

• What’s the difference between a film camera and a video camera. – Film Camera: Color temperature measurement is necessary. – Video Camera: White balance is necessary. (Color temperature measurement is not necessary. • Video camera’s sensor is different from industrial measurement instrument. So video camera cannot measure it precisely. – Color temperature display error doesn’t affect anything to video picture quality because it doesn’t use color temperature. – White balance affect to color reproduction quality.