Color Temperature and White Balance Explained

What You Need to Know

Color temperature and white balance (WB) are how you control (or correct) the color cast of your footage so that whites look white and skin tones look natural.

Core idea (exam-level definition)

• Color temperature (in Kelvin) describes the warmth/coolness of a light source along the blue–amber axis.
• White balance is the camera (or post) process that neutralizes the scene’s illuminant so neutral objects (white/grey) reproduce with equal RGB (no color cast).

Why it matters

• WB affects skin tones, perceived production value, and how well shots match in an edit.
• Bad WB is hard to “fully fix” when footage is compressed or heavily processed (especially in 8-bit codecs).

The two axes you must remember

1. Temperature (blue–amber): measured as CCT in $K$.
2. Tint (green–magenta): not captured by Kelvin; handled via tint control (common with fluorescents/LEDs that skew green).

Critical reminder: Kelvin only describes the blue–amber shift. If your light is greenish, you need tint (or gels like minus-green), not just a different Kelvin.

When you use what

• Use manual Kelvin / custom WB when consistency matters (narrative, interviews, multi-camera).
• Use auto WB (AWB) when conditions change fast (run-and-gun), but expect shot-to-shot shifts.
• Use RAW/Log with planned grading if you want more freedom to adjust WB in post.

Step-by-Step Breakdown

A. Fast, reliable on-set workflow (recommended)

1. Identify your key light source(s)
   • Daylight through windows? Tungsten practicals? Mixed LED panels?
2. Decide your target “world”
   • You usually balance for the subject’s key light (e.g., interview key) rather than the entire room.
3. Set WB using one of these (best to worst):
   1. Custom WB with neutral target (best): grey card / color chart
   2. Manual Kelvin (good): dial $K$ to match the dominant source
   3. Preset (okay): Daylight / Tungsten / Fluorescent
   4. AWB (variable): can drift between shots
4. Check tint
   • If whites are neutral but skin looks slightly sick/green, adjust tint toward magenta.
5. Verify on a trustworthy reference
   • Use a properly set monitor and/or waveform + vectorscope:
     • Neutral areas should cluster near the center of vectorscope.
6. Lock it
   • For a scene, don’t let WB change unless the lighting changes intentionally.

B. Custom white balance (grey card) procedure

1. Place a neutral grey card where the subject is, facing the key light.
2. Fill enough of the frame with the card for the camera’s WB sampling.
3. Trigger custom WB (camera menu) or set WB by sampling (some cinema cameras/monitors).
4. Re-frame, shoot, and don’t move the card into different lighting unless you re-balance.

Mini worked example (decision point):

• Subject lit by a soft LED panel; background has tungsten practicals.
  • Balance to the LED (subject looks correct).
  • Let practicals go warm (often desirable), or gel them if you want neutral.

Key Formulas, Rules & Facts

1) Kelvin and “warm vs cool” (counterintuitive but must know)

• Lower Kelvin = warmer/oranger light (tungsten/candle).
• Higher Kelvin = cooler/bluer light (shade/overcast).

2) Common light sources (typical ranges)

Exam trap: Two lights can share the same “$K$” but still render colors differently because their spectral power distribution (SPD) differs.

3) CCT vs real-world color

• CCT (Correlated Color Temperature): the Kelvin number that best matches a light’s appearance relative to an ideal blackbody.
• LEDs/fluorescents often sit off the Planckian locus, causing tint errors (green/magenta) even when Kelvin seems correct.

4) White balance in-camera (conceptual model)

• WB is effectively applying channel gains (multipliers) so neutral areas become neutral:
  • Increase blue gain to correct overly warm light.
  • Increase red gain to correct overly cool light.
• This is why extreme WB corrections can amplify noise in one channel.

5) Mired (micro-reciprocal degree) and filter math (high-yield for gels)

Color correction filters behave more linearly in mireds than in Kelvin.

Interpretation:

• Positive $\Delta m$ = shift **warmer** (toward lower $K$).
• Negative $\Delta m$ = shift **cooler** (toward higher $K$).

6) WB + recording format (practical rule)

• RAW: WB is metadata (high flexibility).
• Log/rec709 video: WB is largely “baked in” (less flexibility, especially in $8\text{-bit}$).

Examples & Applications

Example 1: Daylight interview setup

Scenario: You’re outdoors in open shade; the camera on Daylight preset looks too blue.

• Shade can be around $8000\,K$ (very cool).
• Fix: Set WB to about $7000\,K\text{--}9000\,K$ (or custom WB on a grey card).
  Key insight: Higher $K$ makes the image warmer (adds amber) to counter the blue shade.

Example 2: Indoor tungsten practicals with daylight window (mixed lighting)

Scenario: Subject near a window (daylight) but room has tungsten lamps.

• If you WB to daylight (around $5600\,K$), tungsten lamps go very orange.
• If you WB to tungsten (around $3200\,K$), the window goes very blue.
  Options (exam-style decision):

1. Pick a side (creative choice): balance for subject’s key.
2. Match sources physically: gel the window with CTO? (rare) or gel tungsten practicals with CTB? (also rare). More common: turn off practicals or swap bulbs.
3. Add your own key: use a daylight-balanced LED and let background practicals stay warm.

Example 3: Fluorescent office looks “green” even at correct Kelvin

Scenario: You dial $4300\,K$ and whites are still greenish.

• Cause: Fluorescent/LED tint spike (often green).
• Fix: Adjust tint toward magenta or use minus-green gel on the fixture.
  Key insight: Kelvin adjustment alone cannot solve green–magenta errors.

Example 4: Using mired shift to understand correction direction

Scenario: You want to convert $5600\,K$ daylight to $3200\,K$ tungsten (warming).

• Compute mireds:
  • $m_{5600} = \frac{10^6}{5600} \approx 178.6$
  • $m_{3200} = \frac{10^6}{3200} = 312.5$
  • $\Delta m \approx 312.5 - 178.6 = 133.9$
    Interpretation: You need a warming correction (positive shift), i.e., a CTO-type move.

Common Mistakes & Traps

1. Confusing “warm” with “high Kelvin”

   • Wrong: thinking $3200\,K$ is cooler than $5600\,K$.
   • Why: Kelvin describes the light source color, not how “warm” the final image feels.
   • Fix: Memorize: Low $K$ = warm light; high $K$ = cool light.

2. Relying on AWB for multi-shot continuity

   • Wrong: AWB shifts between angles/takes, making matching painful.
   • Fix: Lock WB per scene (manual $K$ or custom WB), especially for interviews.

3. Using a white sheet/paper as a WB target

   • Wrong: paper can be dyed or use optical brighteners, pushing WB blue.
   • Fix: Use a neutral grey card or calibrated color chart.

4. Ignoring tint (green–magenta axis)

   • Wrong: chasing the right look only with Kelvin when the real issue is green spike.
   • Fix: Adjust tint, or gel lights (minus-green / plus-green) to match.

5. Mixing sources and expecting one WB to “fix everything”

   • Wrong: balancing for both daylight and tungsten simultaneously.
   • Fix: Decide what should be neutral (usually subject), then manage the other sources (gel/turn off/reposition).

6. Pushing extreme WB correction on compressed footage

   • Wrong: heavy WB changes in $8\text{-bit}$ can cause banding, noise, weird skin.
   • Fix: Get WB close in-camera; shoot $10\text{-bit}$ or RAW when you expect big corrections.

7. Judging WB on an uncalibrated display or wrong viewing mode

   • Wrong: your laptop/phone display lies; camera LCD may be inaccurate.
   • Fix: Use scopes (vectorscope) and a reliable monitor/LUT pipeline.

8. Treating Kelvin numbers as absolute truth across all fixtures

   • Wrong: assuming two $5600\,K$ LEDs will match perfectly.
   • Why: different SPDs and poor calibration lead to different tint/CRI behavior.
   • Fix: Test, use charts, and fine-tune tint; prioritize consistent fixtures.

Memory Aids & Quick Tricks

Quick Review Checklist

• You can explain: color temperature vs white balance in one sentence each.
• You remember: low $K$ = warm light, high $K$ = cool light.
• You know Kelvin does not solve green casts; that’s tint.
• You can list common anchors: $3200\,K$ (tungsten), $5600\,K$ (daylight), $7000\,K\text{--}10000\,K$ (shade).
• You can do mired conversion: $m = \frac{10^6}{T}$ and interpret the sign of $\Delta m$.
• You default to: custom WB with grey card for consistency.
• You avoid AWB when shot matching matters.
• You check WB with vectorscope/neutral reference, not just your eyes.

One clean WB decision per scene makes everything downstream (editing, grading, matching) dramatically easier.