Photometry and Light Measurement

Photometry and Light Measurement Adrian Waltho, Analytik Ltd adrian.waltho@analytik.co.uk

What is Light?

Ultraviolet Light UV-C 180-280 nm UV-B 280-315 nm UV-A 315-400 nm

Infrared Light NIR Near infrared 750-1400nm SWIR Short-wave infrared 1400-3000 nm MWIR Mid-wave infrared 3000-8000 nm LWIR Long-wave infrared 8000-15 000 nm FIR Far infrared 15 000-1 000 000 nm

What to measure? Photometry (Human Perception) or Radiometry (Physical Energy)

Photometry is what we care about today

Human Photopic Curve V(λ) The luminosity function V(λ) describes the average spectral sensitivity of human visual perception of brightness. en.wikipedia.org/wiki/luminosity_function Photopic = Bright light Scotopic = Dim light

CIE XYZ 1931 The CIE's color matching functions x λ, y λ and z λ are the numerical description of the chromatic response of the observer. They can be thought of as the spectral sensitivity curves of three light detectors yielding the CIE tristimulus values X, Y and Z. Collectively, these three functions are known as the CIE standard observer.

CIE 1931 2 degree observer Due to distribution of cones in the eye, the tristimulus values depend on the observer's field of view. To eliminate this variable, the CIE defined a color-mapping function called the standard (colorimetric) observer, to represent an average human's chromatic response within a 2 O arc inside the fovea.

ANSI Chromaticity Binning

Ideal Black Body Radiator Source: VNIIOFI Institute Moscow

Ideal Black Body Radiator hyperphysics.phy-astr.gsu.edu

LEDs are NOT Blackbody Radiators!!! Solid state light sources use Electro-luminescence Blackbody light sources use Thermo-luminescence Lux meters, photometers, colorimeters etc. rely on the predictability of blackbody physics When measuring LED solid state light sources, you can t use instruments designed for blackbody sources Need a Spectrometer! How is the Spectis 1.0 Touch different to a lux meter

Spectrometer Source CCD

Spectrometers measure SPD Spectral Power Distribution Light Power per Wavelength How much blue, green, red light Colour From the SPD we can calculate: Colour (CIE chromaticity co-ordinates) Colour temperature Colour rendering indices Colour quality scale etc. No V(λ) compensation required No Error in calculation New norms and guidelines recommend spectrometers

Spectrometers measure SPD

Spectrometers measure SPD SPD + Geometry = Useful information Brightness and Colour Illuminance (Lux) = SPD + Cosine corrector Luminous flux (Lumens) = SPD + Sphere Luminance (nits or cd.m -2 ) = SPD + Field of View Gonio-spectrometry = SPD + Angular information

WHY Measure LED light? Customers need light measurement at EVERY step in the supply chain

Claimed Specifications Source: Photometry Testing, UK

Measuring light pre-led Relative Photometry Fluorescent lamp output is constant regardless of what fitting it is put into Predictable behaviour Multiply lamp flux by fitting light output ratio (LOR) to calculate flux of luminaire

Measuring LED light Relative Photometry is no longer applicable to solid state lighting LED Output depends on Temperature In turn it depends upon the Thermal design of the fitting For most solid state lighting we have to perform absolute photometry For absolute photometry we need a spectrometer

Measuring LED light Source: Y. Zong, NIST Temperature dependence of luminous flux and colour temperature of a high-power white LED

Measuring LED light Large Gap exists between LED Manufacturers and Lighting Industry Lumen output and Colour temperature are variable and dependent on multiple factors

HOW TO PERFORM LIGHT MEASUREMENT

What is your measurement need?

How to measure LEDs - Standards Lighting Industry Association Technical Statement 01 - LED/OLED Standards & Guidance Lumileds Optical Measurement Guidelines whitepaper GL Optic Knowledge Base

How to measure LEDs - Standards IES LM-79 Electrical and Photometric Measurements of Solid-State Lighting Products Made-do since 2008, de-facto international standard Only North America Lacked worldwide accreditation Other parallel standards active CIE 025/E:2015 Test Method for LED Lamps, LED Luminaires and LED Modules Internationally recognised standard Supersedes LM-79 BS EN 62471:2008 Photo-biological safety of lamps and lamp systems

How to measure LEDs - Lux Fast Simple Versatile Colour temperature Colour rendering Colour co-ordinates (binning) Radiometric power (irradiance vs illuminance)

How to measure LEDs Lumens or

How to measure LEDs Luminous Flux Harder to set up Slower measurement time Larger space needed Colour temperature Colour rendering Full Spatial Information IES / LDT file generation

How to measure LEDs Luminous Flux Easy to set up Fast measurement time Inexpensive Colour temperature Colour rendering NO SPATIAL INFORMATION

Thermal Considerations Anyone who has designed LED products knows that thermal control is essential to lumen output and product stability (Lifetime). IESNA LM-79-08 and IESNA LM-80-08 make reference to controlling temperature on final products: Device Temperature measured at Case Temperature (T s ) Ambient Temperature measured at sphere or within 1m device (for goniometer) and held constant at 25C+/-1C At a minimum, suppliers have to measure Case Temperature of DUT after device stabilization: Stabilization Defined in LM-79: at least 3 readings of light output and electrical power over a period of 30 min. taken 15 minutes apart [with stability] less than 0.5%. This is part of the required report. Ambient Air temperature specified is 25C, but actual environment of device may not be 25C (ex: ASSIST in-ceiling temps of 30-40C). Need to control temp? A more proactive approach is to control DUT temperature (and ambient?) during product development testing and characterization 36

How to measure LEDs Luminous Flux

How to measure LEDs Luminous Flux NIST Uncertainty in Sphere-Based Measurements (Best on Earth) +/-1% Lumens for White LED products +/-0.001 x and y Chromaticity for White LED Products Best Practical Secondary Laboratory Uncertainty +/-1.5-2% Lumens for White LED products +/-0.002 x and y Chromaticity for White LED Products Commercial Laboratories +/-3% Lumens for White LED products +/-0.003 x and y Chromaticity for White LED Products Good Industry Laboratories and QC +/-5% Lumens for White LED products +/-0.005 x and y Chromaticity for White LED Products Practical Production Testing <10% Source: Y. Ohno / NIST, LED 2008 Conference