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LED displays generally fall into two categories: visual displays and displays for marking photosensitive materials. Both types are available as standard products or in custom configurations.
LED OPTICAL PERFORMANCE
Optical performance is normally given as RADIANT INTENSITY (IE) in µW/SR at a peak wavelength λp in nm for displays used for exposure of photosensitive materials. For visual displays the eye response V(λ) must be included. LUMINOUS INTENSITY (Iv) in µcd is normally used for visual displays when far away and the eye does not focus on the LED emitting area. LUMINANCE (lv) is used when close up and focusing on the LED emitting area. For intermediate distances it is hard to say which parameter is best for measuring what the eye sees. Both IE and Iv are relatively independent of the size of the emitting area while Lv is very dependent on the emitting area.
IE, Iv, and Lv are related as follows:
|
Iv = 682 IE • V(λ) Lv = 452 (Iv/A) |
IE (µW/SR) Iv (µcd) Lv (Ft.L) A (mil2) |
For an overview of radiometric and photometric relationships click here.
Click here to download a FREE program to perform these Light Measurement conversions.
MONOLITHIC LED DISPLAYS
Optotek designs and manufactures a wide range of custom LED displays both for visual applications and for exosure of photosensitive materials. Many of these displays have integral drive and interface electronics. The custom ICs used can provide both Amplitude Modulation (AM) and Pulse Width Modulation (PWM) which can be used for LED uniformity correction and grey scale. Linear LED arrays have resolutions up to 600 DPI and matrix resolutions up to 100 DPI. These monolithic displays are made in GaAsP and in GaAlInP.
Typical Monolithic LED Characteristics
| Peak Wavelength Range λp | 660 nm | 680 nm | 590 nm | 630 nm |
| Color | Red | Red | Amber | Red |
| Minimum Emitting Area | 25µm x 25µm | 25µm x 25µm | 35µm x 35µm | 35µm x 35µm |
| Radiant Intensity IE @ 10mA | 10µW/SR | 10µW/SR | 50µW/SR | 50µW/SR |
| Luminous Intensity Iv @ 10mA | 417µcd | 116µcd | 25.9mcd | 9.1mcd |
| Spectral Half Width | 30nm | 30nm | 15nm | 15nm |
| Wavelength Change | +0.2nm/° C | +0.2nm/° C | +0.15nm/° C | +0.2nm/° C |
| IE Change | -0.5%° C | -0.5%/° C | -0.5%/° C | -0.5%/° C |
| Forward Voltage Vf @ 10mA | 1.6 volts | 1.6 volts | 3.5 volts | 3.5 volts |
| Uniformity IE Hi/Lo | 2:1 | 2:1 | 2:1 | 2:1 |
IE is relatively independent of LED emitting area, but does change some with diode size and contact configuration.
HYBRID LED DISPLAYS
Hybrid LED displays are assembled using individual LEDs with typical dimensions of 300µm x 300µm (12 x 12 mils). The resolution of both linear and matrix arays is therefore limited to somewhere between 60 x 80 DPI. However, a wide range of more efficient LED materials can be used covering a wide range of wavelength from ultraviolet (UV) to infrared (IR). Radiant intensity (IE) uniformity is typically Hi/Lo ≤ 2 before drive electronics compensation.
Typical Hybrid LED Characteristics @ 10mA
| Peak Wavelength Range λp (nm) | 565 | 665 | 380 | 470 | 530 | 605 | 660 | 720 |
| Color | Green | Red | UV | Blue | Green | Orange | Red | NIR |
| Radiant Intensity IE (µW/SR) | 3 | 10 | 80 | 400 | 200 | 130 | 220 | 240 |
| Luminous Intensity Iv (mcd) | 2.0 | 0.3 | 1.0 | 25 | 120 | 50 | 9 | 0.2 |
| Spectral Half Width | 30 | 30 | 15 | 15 | 15 | 15 | 20 | 25 |
| Wavelength Change (Ang/° C) | 1.0 | 1.0 | - | - | - | 1.5 | 1.5 | - |
| Forward Voltage Vf (volt) | 2.5 | 1.8 | 3.9 | 3.8 | 3.8 | 3.5 | 3.5 | 1.7 |
| Optotek Reference No. | 7 | 6 | 1 | 2 | 3 | 4 | 5 | 6 |
