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Radiometry and the Radiometer • Illumination/Electrical Group
Radiometry • Study of measuring and characterizing electromagnetic radiation (light). • RADIOMETRY VS. PHOTOMETRY • Radiometry studies light in a way that takes into account more than just visible light. It is measured so that all wavelengths are looked at evenly. • Photometry focuses on the visible portion (400-700nm) of the electromagnetic spectrum. It peaks where human eye sensitivity peaks, and is generally thought of as the measurement of how humans perceive light.
Measurements • Each have their own terms and units. (boxes horizontal to each other are the same measurement type, but may have a different name for each radio/photometry.
What do these mean? • Flux measures power output of a light source, or the rate of flow of the photons out of the light source. Radiometric is in Watts [W] which can be converted to the photometric measure of lumens [lm]. Lumens can not be converted back to watts. • In Radiometry, irradiance measures the amount of power per unit area of electromagnetic radiation at a surface. In other words: how much power from a light source is received by one unit area of a surface [W/m^2]. • Illuminance measures the same in photometry. Mostly commonly known as Lux [Lm/m^2] • Intensity is the number of photons being emitted from a source. Measured in Watts per Steradian [W/sr]. in radiometry and in photometry measured in lumens per steradian [Lm/sr], also known as a candela (“candle power”) [cd]. Steradian is the the solid angle of a beam of light. Figure below shows a comparison to Radians, which is used in planar circles.
(Terms Continued) • Radiance, in radiometry, is the infinite amount of radiant flux (power) in a ray of light that interacts with a surface. Measured in [W/sr*m^2]. • Luminance measures the same but just for visible light and it is in [Lm/sr*m^2] or [cd/m^2].
How can we measure these quantities? • RADIOMETER! • Similar to Light Meter but more advanced, more accurate, more complicated, and reads beyond visible range. • Uses a detector with different filter combinations to read values of each quantity. • Reads radiance, irradiance, radiometric intensity, luminance illuminance, photometric intensity, watts and lumens, and has separate filters for radiometric and photometric measurements.
How does it work? • Put appropriate filters (blue for radiometry, green for photometry) onto detector to get quantity you want to measure • Point it at light source (or source where light is reflecting even) • Value shows up in screen that says “x10” next to it. Comes out in scientific notation. shows number (with a plus sign to indicate positive), times 10, and an exponent above the 10 to show power (may be negative or positive)
Why Are These Numbers Not Matching what we expected? • Needs to be CALIBRATED for every quantity! • Each device made has its own little flaws, they are not all made the same. They are all tested individually in a controlled environment on lights with known, and measured characteristics (NIST standard). • Values were off, so they looked at the difference in values, and used that difference as the “calibrating coefficient.” these values are different for every device.
How Do You Calibrate It? • First, change one switch from “Data Display” (when it takes readings) to “Factor Shift,” so you can calibrate it. • Can enter up to 10 calibrating coefficients (factor numbers 0-9) so you don’t have to calibrate it for every measurement. Enter number given on the sheet corresponding to the measurement type in the following way: • Hit EXP button to change exponent on the “x10” to what the exponent is in the cal. coeff. • Hit MSD (most significant digit) to change number to the left of the decimal, and LSD (least “ “) to change numbers to the right of it. • Switch back to “Data Display.” Cal. coeff. you entered is now “factor number 0. • Calibration done! • Needs to be repeated for every type of measurement with each value given.
Zero the Detector? • Absolute Zero: zero when there is no light entering the detector. Cover lens with opaque object (one that won’t allow light through, the black cover given will suffice) and hold ZERO for over one second. Used to make an accurate specific reading of a light. • Ambient zero: zero when there is no cover on, or at a certain point when light enters detector. Just hold ZERO when there is no cover on, and it will zero the detector at whatever value it just last read. Any other readings taken will tell you the difference from that value to the one taken when the detector was zero-ed.
Filter Combinations: Radiometric (Put flattening filter on first!) • Irradiance: • Cosine Corrector filter on top of flattening filter (same for every other filter) => 13238 • White diffuser with dome • use units for irradiance when reading value on screen. • For example: if it says 9.85 x10^-4, the units are [W/cm^2] (in cm^2 instead of m^2 because those are the units the calibration certificate says it is in) • do this for the respective units of other measurements • Radiance: • [W/sr*cm^2] • Radiance Barrel => R#1164 • Creates a solid angle by its small-to-large lens alignment • Radiometric Intensity: • [W/sr] • LED Barrel => 0026 • Longer barrel, specifically measures intensity of LEDs (hole fit for them) • Forms solid angle to measure wattage distribution over the angle.
Filter Combinations: Photometric(Put on green filter!) • Illuminance: • [Lux] = [Lm/m^2] • Cosine Corrector (same as Irradiance) • Luminance: • [cd/m^2] = [Lm/sr*m^2] • Radiance Barrel • Photometric Intensity: • [Lm/sr] = [cd] • LED Barrel
Power Measurements • Must remove area part of Irradiance or Illuminance measurement. Example: [W/cm^2], we just want [W]. • Use Pen-Probe detector =>SPD024F (radiometric, in [W]) • White cavity inside it integrates light to eliminate spatial and geometric properties, concentrates light evenly and makes it uniform so detector can read one power or radiant flux value • Has a mini-integrating sphere inside it. • Integrating spheres receive light inside, bounce it off the surface inside countless times until it exits out another hole in uniform, gets rid of “flashlight beam” formation, so there is no uneven distribution of the radiant flux over the area anymore, it is all released over the entire area of the ray of light. • Exit hole is where the detector goes, and it takes a uniform measurement of the flux. • For photometric radiant flux [Lm], use SPD024Y
How can we use this? • Verify the specs we are given on the websites for the lights we bought from them when testing lights. • Determine how lights will behave in PTM dome and how it will interact with the object and in turn the camera. • Makes testing much more convenient.