NFMS THEORY LIGHT AND COLOR MEASUREMENTS AND THE CCD-BASED GONIOPHOTOMETER Presented by: January, 2015 1
NFMS THEORY AND OVERVIEW Contents Light and Color Theory Light, Spectral Power Distributions, and the Photopic Curve Color and Color Measurements with Filter Based Devices Luminance (near field) and Luminous Intensity (far-field) Goniophotometers and Near Field Measurement Systems NFMS Alignment Overview ProMetric Calibration Overview 2
LIGHT AND COLOR THEORY A look at spectrums and units 3
WHAT IS LIGHT? Electromagnetic Radiation EM Spectrum of Light extends from UV to IR Visible light consists of a very narrow band between UV and IR Spectrum of light can be broken into discreet wavelengths that we see as colors 4
LIGHT SOURCES AND SPECTRAL POWER DISTRIBUTIONS (SPD) Light emitted by a source can be defined by the source s Spectral Power Distribution (SPD) SPD is the light output (power) at a given wavelength along the EM spectrum Different sources will have different SPD s Incandescent Lamps Florescent Bulbs LEDs Lasers 5
Power SPD OF COMMON LIGHT SOURCES SPDs of Common Light Sources 1 0.8 0.6 0.4 0.2 Illuminant A (Lightbulb) Illuminant D65 (Sunlight) Metal Halide Lamp LCD RED LCD Green LCD Blue LED Red 0 Wavelength nm 6
HUMAN VISION Human eyes do not see entire SPDs, only color and brightness. Three types of cones, the photoreceptors in our eyes, differ in their absorption characteristics. Light with a particular SPD will stimulate some cones more than others, which our brains interpret as brightness and color. Different SPD s may appear the same to human eyes. 7
PHOTOPIC V. RADIOMETRIC This is the overall response of the eye to each wavelength of light. 8
COLOR AND THE CIE TRI-STIMULUS CURVES CIE coordinates are calculated by taking the integrated product of the Tristimulus Curves and the SPD to calculate X, Y, and Z. These values are then plotted using the defining equation for each CIE Chart. 1931 CIE x = X/(X+Y+Z) y = Y/(X+Y+Z) 1976 CIE u = 4X/(X+15Y+3Z) v = 9Y/(X+15Y+3Z) 9
LUMINANCE AND LUMINOUS INTENSITY Luminous intensity is defined to be the luminous flux emitted by a source in a given direction (candela or Lumens/steradian). Luminous intensity distribution is this quantity as a function of angle I(q,f). Luminous intensity is a far-field measurement where it treats the source as a point source. 10
LUMINANCE AND LUMINOUS INTENSITY Luminance is defined to be the luminous flux emitted per unit solid angle per unit area of a source (candela/m 2 ). Luminance is a measurement of the angular and spatial distribution of source output. Luminance is measured in the near field; that is, at a distance from the source small enough so that any source structure is apparent. 11
KNOWING THE UNITS Quantity Measured Photometric Radiometric Total light output Luminous Flux Lumens (lm) Radiant Flux Watts (W) Light incident on a surface Light from a Direction Brightness Illuminance Lux 1 Lux = 1 lumen/meter 2 Luminous Intensity candela 1 candela (cd) = 1 lumen/steradian Luminance cd/m 2 or nit 1 nit = 1 candela/meter 2 Irradiance (W/m 2 ) Radiant Intensity W/sr Radiance W/sr * m 2 12
LUMINANCE AND LUMINOUS INTENSITY Luminous intensity can be calculated from a set of luminance measurements taken from various angles around the source. This is done by generating rays from the set of near field (luminance) measurements and then retracing to the far-field. Both NFMS and ProSource can do this conversion. 13
GENERATING RAYS Rays sets can be created from Radiant Source Models These are made by taking luminance measurements at many angles and then tracing the position and angle of the luminance. If spectral data is available, it can weight the color to the SPD to create rays which are both specially and spectrally accurate. 14
GONIOPHOTOMETERS AND NEAR FIELD MEASUREMENT SYSTEMS An Overview of the two systems 15
Goniophotometers and Near Field Measurement Systems Goniophotometer (traditional measurement) Source mounted on a two axis goniometer Stationary photometer placed in far-field Source is rotated in two axes, allowing entire intensity distribution to be sampled Luminous intensity distribution created from point measurements Alternate configuration is moving mirror goniometer DUT Spot meter 16
Goniophotometers and Near Field Measurement Systems NFMS Source mounted on a two axis goniometer Stationary imaging colorimeter placed in near-field views DUT directly Source is rotated in two axes, allowing luminance measurements to be recorded at all angles Software produces near-field model of luminance and chromaticity vs. angle Ray tracing is used to yield illuminance or far-field intensity data DUT ProMetric Colorimeter 17
Goniophotometers and Near Field Measurement Systems Spot Goniophotometer Advantages Color and luminance accuracy Disadvantages Slow speed LED output can vary during measurement Provides luminous intensity only at measured distance Large space required for extended sources 18
Goniophotometers and Near Field Measurement Systems NFMS Advantages Delivers comprehensive data Measures in the near field (compact configuration) Results compatible with optical design software Measures luminance and color characteristics of approximately 100 mm to 1.6 m long sources Disadvantages Slow speed Source output can vary during measurement Only measures luminance intensity for small sources 19
ALIGNMENT OVERVIEW Methods for perfecting Camera/Goniometer placement 20
ALIGNMENT Matched Height It is important for the camera to be at the exact same height as the center of rotation as this allows the camera to be normal to the plane of rotation. 21
ALIGNMENT Matched Horizontal position The camera should be approximately centered on the horizontal. This is not as critical because both the NFMS and Camera can rotate on the horizontal plane. 22
ADDITIONAL ALIGNMENT The NFMS only needs to be leveled if it has been moved. Using a spirit level or similar instrument, level the NFMF. You can fine tune the ProMetric Colorimeter to match the tilt of the NFMS using the live image. 23
ALIGNMENT Use the Active Align mode to aim the camera at the crosshairs. 24
LASER ALIGNMENT Now that the camera is pointed at the NFMS and everything is leveled, we need make the NFMS face normal to the camera. Begin by turning on the laser 25
LASER ALIGNMENT Next, point it at the secondary crosshairs. Note: The offset from the camera lens to the laser is the same as the offset between the two crosses on the plate. 26
LASER ALIGNMENT Then flip the secondary crosshairs over to reveal the mirror. 27
LASER ALIGNMENT Adjust the NFMS using the software to retro-reflect the laser back onto itself. Set home then enter the mirror off sets. Set home again, and the system is now aligned. 28
ADDITIONAL ALIGNMENT The NFMS only needs to be leveled if it has been moved. Verify with rotation Level the second rotational axis of the NFMS by rotating the NFMS horizontally and adjust the forward/backward tilt to ensure the rotation axis remains level as it moves towards and away from the camera. Adjust tilt 29
MOUNT YOUR SOURCE You are now ready to mount your source. 30
Contact: info@radiantvs.com 31