Improved Radiometry for LED Arrays

Personal Background Studied photochemistry of 1 st row transition metal complexes Used visible monochromatic sources (Ar +, Kr +, and doubled Nd-YAG laser) Post-doctoral in organometallic photochemistry in Regensburg Used filters/monochromator to select narrow spectral bands Joined 3M s Central Research Laboratory in 1979 in St. Paul, MN Used broadband medium-pressure arc and microwave mercury lamps Radiometry was radically different with no established standards Cured using excimer ( 93) and low-pressure Hg lamps ( 96) and LEDs ( 01) Radiometry using a narrowband source is straight forward and more believable Cure gradients based on absorbance can be calculated/evaluated 3M 2017 All Rights Reserved. 2

Medium-Pressure (MP) Mercury Lamp Spectrum MP mercury lamp UV emission spans from ca. 220 to 400 nm Filters are used to isolate different spectral bands (UVA, UVB, UVC) Transmission curves are not standardized between radiometer manufacturers Calibration using a MP Hg lamp is required but what is a standard MP Hg lamp? Spectrum can vary due to temperature, quartz type, reflector composition, additives, etc. Generic mercury bulb spectrum 3M 2017 All Rights Reserved. 3

Light-Emitting Diodes (LEDs) LEDs emit within a narrow spectral range ( max ±25nm) LED spectra are similar to excimer lamp spectra and can be considered as pseudo-monochromatic sources Filters to isolate LED emission bands are not necessary Arrays of >340 nm can exceed peak irradiances of 50 mw/cm 2 Background lighting can often be neglected XeCl excimer lamp Ensemble of discrete electrical discharges max = 308 nm LED array Ensemble of individual LEDs max = 395 nm 3M 2017 All Rights Reserved. 4

Conventional UV Radiometers UV radiometers use Si photodiodes Advantages Fast response time Good linearity Disadvantages Saturate at high power Sensitivity degrades with UV exposure UV responsivity decreases as temperature increases Spectral responsivity is strongly dependent on wavelength Calibration recommended every 6-12 months 3M 2017 All Rights Reserved. 5

Problem Statement There are no standard methodologies for measuring radiant power impinging on a surface (irradiance) from LED arrays Results can vary dramatically depending on the distance from the source, the array configuration, and the detector responsivity Use of Hg spectral band definitions (UVA, UVB, UVC, etc.) makes no sense when measuring narrowband LED arrays 2014 NIST study requested by RadTech showed that measuring the same LED source using two radiometers from different suppliers resulted in a 30% difference in the reported peak irradiances 3M 2017 All Rights Reserved. 7

Lasers LED spectra are more similar to lasers than MP Hg lamps Lasers are coherent sources; LEDs are incoherent sources Laser power is measured using power meters Power meters use sensors that absorb radiation in combination with a thermopile 3M 2017 All Rights Reserved. 9

Thermopile Technology Thermopiles convert thermal energy into electrical energy Output is proportional to a local temperature difference or gradient Advantages Flat spectral response High saturation threshold Disadvantages Slow response time Sensitive to ambient temperature changes Could thermopile technology be used with LED arrays? 3M 2017 All Rights Reserved. 10

Use of PowerMax-Pro with LED Sources Static LED Measurements Reported by Coherent, Inc. Measured performance characteristics of LED s for resin curing Typical rise-time Optical measurements in 10 µsec Measurement insights Improve battery and diode lifetimes Insure ISO (10650-1) or FDA compliance Adjust exposure parameters (power & energy) for optimum resin curing We love it! It is the first time we ve actually been able to see the needed resolutions on our output and will now be able to verify and validate tracing directly back to a NIST traceable meter. This is big for us! Customer comment 3M 2017 All Rights Reserved. 12

Coherent PowerMax-Pro vs. Conventional Thermopiles Transverse Thermoelectric Physics conventional Sensor Response Time Atomic layer thermopiles PowerMax-Pro t rise Diameter 2 Conventional Thermopile t rise (Thickness) 2 Coherent Thermopile Could this technology be adapted to be a profiling radiometer? 3M 2017 All Rights Reserved. 14

Attributes of PowerMax-Pro PowerMax-Pro uses an anisotropic thermoelectric sensing element that eliminates the negatives of conventional thermopiles Advantages High sensitivity Fast response time ( 10 µsec) Saturation threshold in excess of 200 W/cm 2 Flat responsivity curve 20 khz sampling rate Option to water cool for continuous exposures Disadvantages New and unproven 3M 2017 All Rights Reserved. 15

Use of PowerMax as a Profiling Radiometer for LED Arrays Coherent demonstrated a static response using LED spot sources Question - could it be used as a profiling radiometer for LED arrays? Discussed concept with NIST and received favorable feedback no obvious pitfalls Negatives of original Coherent device Sensor is tethered to a controller by a 2m cable Sensor is recessed 0.95 cm from top surface Total height of sensor was 33 mm Reflectance from sensor surface was a concern Original Coherent sensor Inquired about use of a black body absorber as a sensing element to minimize dependence 3M 2017 All Rights Reserved. 16

First Generation Black Body Absorber Black Body (BB) Absorber vs Si Photodiode Photoresponsivity Curve Normalized Responsivity at 400 nm Responsivity Curves (340-460 nm) BB Absorber Si Photodiode BB Absorber Si Photodiode Data provided by Coherent, Inc. 3M 2017 All Rights Reserved. 17

Original and Low Profile (LP) Black Body Sensors Evaluated Black Body (BB) absorber in original sensor head BB original and low profile sensor with 1.0 cm 2 aperture Edge view of LP and original heads (height = 13 mm vs. 30 mm) 3M 2017 All Rights Reserved. 18

Single Pass Video Used LED flashlight as source Setup: Trigger = 10 mw Pre-trigger = 1000 Buffer capacity = 10000 Sampling rate = 20 khz X-axis is time; Y-axis shows irradiance 3M 2017 All Rights Reserved. 21

Summary A laser power meter from Coherent has been successfully used as a profiling radiometer for LED arrays with promising results A low-profile version (13 mm height) using a black body absorber has been tested Suggested improvements Eliminate tether from detector to controller Add internal battery Provide onboard data storage and wireless data link Current status Coherent is evaluating production, stability, and business opportunity 3M 2017 All Rights Reserved. 30