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Photometry, or colorimetry, is…

Lecture 1: Photometer

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… …

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And the reason we came here … Simple photometer

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Simple photometer LED applications

• Indicator • Automobile tail lights • Low‐power flashlights • Home lighting? • Remote controls • Optical isolation • Mini fluorescent microscopes

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LED properties ‐ mechanical From the LED catalog pages: Note flat part of rim • can be broad or narrow spectral range – White covers 420‐700 nm Cathode (-) – 428 and 470 nm are common short‐ values  – Special purpose < 400 nm but > $5 each

Anode (+) • IR emitters at 880 & 950 nm are common – Long lambda: GaP, GaAlAs – Mid (500s): GaAsP, InGaAlP Spherical or parabolic lens 3mm (T-1) or 5 mm (T-1 ¾) – Blue : GaN in diameter – White: InGaN

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From the LED catalog pages: LED properties –optical.1 • Output angle varies from 8°– 75° • Most hemispherical but some tapered/parabolic.

• Package can be clear or colored Reported in data sheet as • Forward current 15 –40 mA, fwd voltage 1.7 –3.9  = +/- 4  • Lig ht output from 6 to 9300 mcd (m illican de la )

Source: Vishay Semiconductors, THLB580x data sheet

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LED properties –optical.2 LED properties – electrical.1 0.80 0.70 Diode v-i curve 0.60 Rated for 20 mA 0.50 0.40 0.30 0.20 forward current, A current, forward 0100.10 0.00 0 0.2 0.4 0.6 0.8 forward voltage, V

Typical curve for Si diode + – Source: Vishay Semiconductors, THLB580x data sheet Source: Vishay Semiconductors, THLB580x data sheet i 9/30/2009BIOEN 302 13 9/30/2009BIOEN 302 14

LED properties –optoelectronic Photometric terminology • • 683 lumens = 1 Watt of Note: power emitted or Normalized to 20 mA. received as visible light 10x increase in current produces < 2x increase in output. • Luminous intensity • 1 candela = 1 lumen/steradian (lm/sr)

Source: Vishay Semiconductors, THLB580x data sheet

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Photodetectors applications

• Photocell • Motion sensors (resistance decreases with light) • Daylight sensors • • Optoisolation (current increases with light) • Transmittance measurement • Phototransistor (composed of two ) • Voltage is applied across leads, light controls the current

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Phototransistor: General transistor properties size and shape Source: BPV11 data sheet collector • VC > VB > VE • VB –VE determines base current • In phototransistor, base is not connectdted and emitter incident light determines current • C, B, E can appear in any order on the physical component

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Phototransistor: current vs. light level Phototransistor: Sensitivity

Note radiometric (not photometric) quantity

Source: Vishay Semiconductors, BPV11 data sheet Source: Vishay Semiconductors, BPV11 data sheet

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LED driver: current source Detector circuit +5V

• Similar to voltage divider light • May be buffered or amplified – When might each of these be useful? • Calibrate output vs. concentration LED V-out

Control voltage ground • Voltage‐controlled current • Voltage regulation with source using operational zener diode; current control

amplifier, iLED = V1/R1 with potentiometer

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Mechanical considerations Prelab questions

• Maximize the portion of the light that • What are limits imposed by op‐amp or other goes through the liquid in the cuvette circuit components? • How do we test for these limits? • Prevent stray ambient lig ht • To be discussed in lab… – Derivation of LED current  Design cuvette holder in BIOEN 304 – Op‐amp limitations

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