COLORIMETRY VS PHOTOMETRY
Colour vs Quantity — photometry measures how much visible light is produced or received, while colorimetry measures the colour characteristics of that light. In AGL, both are important: photometry verifies intensity and distribution, while colorimetry verifies that the emitted light has the required colour characteristics.
At a Glance
| Feature | Colorimetry | Photometry |
|---|---|---|
| Main purpose | Measures the colour of light | Measures the quantity and distribution of visible light |
| Main question | What colour? | How much light? |
| Typical quantities | Chromaticity coordinates, colour characteristics | Luminous intensity, illuminance, luminance |
| Common units | Chromaticity coordinates / colour parameters | cd, lx, cd/m², lm |
| Beam distribution / directionality | Not applicable | Yes |
| Colour verification | Primary purpose | Limited alone |
| AGL use | Verify required aviation light colour | Verify optical output and beam performance |
What Each One Measures
Colorimetry
Concerned with the colour characteristics of light. It provides an objective method for describing colour rather than simply relying on visual judgement.
- Is the light red, green, white or blue?
- Does the measured colour fall within the required limits?
- Expressed through chromaticity coordinates
Photometry
Concerned with the quantity of visible light as perceived by the human eye — how intense it is, how it is distributed, and how much illumination it produces.
- Luminous intensity → candela (cd)
- Illuminance → lux (lx)
- Luminance → cd/m² · Luminous flux → lumen (lm)
Quantity vs Quality
Photometry and colorimetry describe two entirely different aspects of the same emitted light.
Easy memory: Photometry = QUANTITY of light. Colorimetry = QUALITY/COLOUR of light.
A Green Taxiway Centreline Light
Consider a green taxiway centreline light. Photometry and colorimetry each examine a different question about the same fixture.
Photometry Checks
- Intensity, in candela
- Beam distribution
- Horizontal coverage
- Vertical coverage
Colorimetry Checks
- Is the emitted colour green?
- Does its measured chromaticity fall within the specified colour limits?
Therefore, a light can be correct colour but insufficient intensity, or high intensity but incorrect colour — both characteristics may need verification.
Directional Intensity Data
Suppose a fixture produces the following intensity in different directions. This describes photometric performance — it answers "how much light is emitted in each specified direction?"
| Direction | Intensity |
|---|---|
| Centre beam | 10,000 cd |
| +5° | 7,000 cd |
| +10° | 2,000 cd |
Two Lights, Same Apparent Colour
Two lights may both appear approximately green to the human eye, yet have slightly different spectral and chromaticity characteristics. Colorimetry provides measurable data to distinguish them.
Visual Colour Inspection Is Not Enough
An inspector may say "the light appears green." However, perceived colour can be affected by several factors that a colorimeter is not subject to:
- Ambient light
- Viewing conditions
- Observer position
- Individual colour perception
- Background brightness
Colorimetry provides a more objective measurement of a light's colour characteristics than visual judgement alone.
What Each Measurement Misses
Photometry Doesn't Define Colour
A light can have a high luminous intensity — say 10,000 cd — but that number alone does not tell you whether the light is white, red, green or blue. Two different readings of 8,000 cd could theoretically represent lights of different colours.
Colorimetry Doesn't Define Intensity
Colour information alone does not tell you how much light is emitted. Two lights with correct, matching colour characteristics can still have very different photometric output — one at 10,000 cd, another at only 3,000 cd.
Photometry vs Colorimetry in AGL
| AGL Characteristic | Photometry | Colorimetry |
|---|---|---|
| Luminous intensity | Yes | No |
| Beam distribution | Yes | No |
| Horizontal coverage | Yes | No |
| Vertical coverage | Yes | No |
| Illuminance | Yes | No |
| Luminance | Yes | Not primarily |
| Colour identification | Limited alone | Yes |
| Chromaticity verification | No | Yes |
| Aviation colour compliance | Supports optical assessment | Primary measurement |
How Each Is Measured
Photometric Measurement
A photometer or photodetector measures the light and returns intensity data.
- Result in cd
- Result in lx
- Result in cd/m²
Colorimetric Measurement
A colorimeter or spectroradiometric measurement system measures the light and returns colour data.
- Measured chromaticity coordinates
- Colour parameters against specified limits
The exact instrument depends on the applicable measurement method and required accuracy.
Checks by Fixture Type
| AGL Light | Photometry Checks | Colorimetry Checks |
|---|---|---|
| Runway edge light | Intensity and beam distribution | White/yellow colour characteristics |
| Taxiway edge light | Light output | Blue colour characteristics |
| Taxiway centreline light | Intensity distribution | Green colour characteristics |
| Runway threshold light | Directional intensity | Green colour characteristics |
| Runway end light | Directional intensity | Red colour characteristics |
| PAPI | Beam/intensity characteristics | Red and white colour characteristics |
Myths vs Facts
Brightness does not verify colour. A fixture can have ample intensity while still falling outside the required chromaticity limits.
Correct colour does not guarantee sufficient intensity or beam coverage. A correctly coloured light can still fail an intensity or distribution requirement.
Photometry and colorimetry describe different aspects of optical performance. Neither one, alone, fully characterizes an AGL fixture.
Colorimetry vs Photometry
Photometry and colorimetry are complementary, not substitutable. Photometry checks whether an AGL light is strong enough and correctly distributed; colorimetry checks whether the emitted colour meets the required characteristics.
Neither measurement alone provides a complete optical performance check. A light can be the correct colour but insufficiently intense, or intense enough but the wrong colour — for full AGL compliance, both must be verified.
For AGL: think of photometry when discussing how much light is emitted and how it is distributed, and colorimetry when discussing whether the emitted colour meets the required aviation colour limits.
Engineering & Expert Notes
Measure Both Where Required
ScopeA light can pass intensity and fail colour or the reverse. Check each requirement.
Use Instruments for Their Purpose
EquipmentA photometer does not verify chromaticity. Use the right instrument.
Track Colour Over Time
AgeingColour can shift with age and temperature. Compare with the baseline.
Report Coordinates and Conditions
ReportingRecord the measured values and the test conditions.
Frequently Asked Questions
Colorimetry → COLOUR (what colour is the light). Photometry → QUANTITY (how much light, and how it is distributed).
Yes. A fixture can have correct colour but insufficient intensity, or high intensity but incorrect colour. The two checks are independent, so both are typically required.
Perceived colour can be affected by ambient light, viewing conditions, observer position, individual colour perception and background brightness, so a formal colorimetric measurement is more objective than a visual judgement.
No. Candela tells you about luminous intensity, not the complete colour specification. Two lights with similar candela ratings could, in principle, have different colours.
Photometry typically uses a photometer or photodetector to produce intensity data in cd, lx or cd/m². Colorimetry typically uses a colorimeter or spectroradiometric system to produce chromaticity/colour data.
Both characteristics are commonly relevant since AGL fixtures typically carry both an intensity/distribution requirement and a colour requirement. Always confirm exactly which checks the applicable specification requires for a given fixture.
Governing Documents
For an actual project design, always verify the current edition of the applicable documents.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — sets out both the photometric intensity requirements and the colour/chromaticity requirements for aeronautical ground lights.
FAA AC 150/5340-30
Design and Installation Details for Airport Visual Aids — photometric and colour requirements relevant to interpreting AGL fixture performance.
CIE Colorimetry & Photometry Standards
International Commission on Illumination definitions and measurement methods for chromaticity coordinates alongside luminous intensity, illuminance and luminance.
Final Engineering Note
Do not treat a photometric pass as a colour pass, or a colour pass as a photometric pass. Confirm which measurement the governing standard requires before selecting an instrument or acceptance criterion.
Photometry measures how much visible light is produced or received; colorimetry measures the colour characteristics of that light — for AGL compliance, both checks are complementary and generally both are required.
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