Every lighting product that needs an IES or LDT file has to pass through a goniophotometer. What's less obvious is that "goniophotometer" isn't one instrument design; it's a family of measurement geometries, each built around a different physical motion.
What a Goniophotometer Actually Measures
A goniophotometer determines luminous intensity distribution across all directions, producing the IES or LDT file lighting designers import into tools like DiaLux. How the instrument achieves angular sampling — what physically moves, what stays fixed — separates the goniophotometer types.
Type C: Mirror and Detector Move, Luminaire Stays Fixed
Type C is the dominant configuration for general lighting testing: the luminaire remains completely stationary while a rotating mirror and detector assembly moves around it. This matters for LED thermal behaviour — output and colour shift measurably with junction temperature, influenced by orientation. A goniophotometer that physically rotates the luminaire disturbs thermal equilibrium, introducing error. Type C's fixed-luminaire design eliminates this, which is why LM-79 effectively requires it for compliant testing.
Type B: Luminaire Rotates, Detector Stays Fixed
Type B works the opposite way — the photometer head stays static, and the luminaire rotates on a turntable. This is the traditional approach for floodlighting, though it's been progressively displaced by Type C for LED-specific compliance testing given the same thermal disturbance issue.
Type A: The Automotive and Signal-Lamp Specialist
Type A is built specifically for automotive headlamps, tail lamps, and signal lighting, with the sample rotating around both axes similar to Type B, built around the specific angular ranges and regulatory test points required by FMVSS, ECE, and similar transport lighting standards. Not a general-purpose instrument.
Comparing the Three Types
| Factor | Type C | Type B | Type A |
|---|---|---|---|
| What moves | Mirror + detector | Luminaire | Luminaire |
| LED thermal accuracy | Best — no orientation disturbance | Lower | N/A |
| Primary standard | LM-79, CIE 121 | Legacy floodlighting methods | FMVSS, ECE |
| Typical application | LED bulbs, downlights, street lights, general lighting | Floodlights, high-bay (legacy) | Automotive headlamps, signal lamps |
Near-Field vs Far-Field Variants
Near-field Type C instruments position the detector close to the luminaire, suited to compact products without requiring large darkroom dimensions. Far-field configurations, with longer test distances, are needed for larger luminaires like street lights, where a close-range measurement would introduce error from angular subtense. Lab space is frequently the deciding factor.
Choosing for Your Lab
For a lab primarily testing LED luminaires, Type C is the near-mandatory answer given LM-79 compliance requirements. For labs spanning both general and floodlight testing, a convertible Type C instrument with a Type B accessory kit avoids two separate installations. For automotive or transport lighting, Type A is a distinct specialist purchase driven by the regulatory standards. The LISUN LSG-6000 is a representative Type C moving-mirror goniophotometer.