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Sunlight Readability Contrast Measurements for the ISD2351 and ISD2353 Serial Input Small Alphanumeric Display Appnote 47
by Bob Krause Introduction Light emitting diode alphanumeric displays have had a long and successful relationship with military and avionic equipment. Applications with very high light ambients now are possible because of advances in the efficiency of LEDs. Polarized filters enhance the readability of LEDs by eliminating much of the ambient reflection from the LED die and the surrounding package. Readability Criteria An observer's ability to perceive the information from an alphanumeric display depends on two factors. These include font and size of the message in relationship to the viewing position and the optical contrast of the message to the surrounding environment. Optical contrast is the comparison of the brightness or sterance (L) of the On/Off/LED to the brightness of the surrounding environment. High readability results by optimizing the following contrast ratios. The first ratio, C1, involves the OnLED stearance, LLEDon, and the background stearance, LB, is optimized when the OnLED is brighter than the surrounding area. Thus C1 will be much greater than unity (1). The next ratio, C2, involves the sterance ratio of the On, LLEDon, and Off, LLEDoff, LED. C2 is optimized when the LLEDon is much greater than the LLEDoff, resulting in being much greater than unity (1). The optimal ratio of C3, involving OffLED to the background, is achieved when the ratio is near zero. The OffLED should have minimal contrast with the background. Ratio Equations 1) C1--OnLED to Background L LEDon - L B C 1 = -----------------------------LB 2) C2--OnLED to OffLED L LEDon - L LEDoff C 2 = -------------------------------------------L LEDoff 3) C3--OffLED to Background L LEDoff - L B C 3 = --------------------------------------LB The equations for these three ratios are given below. The U.S. military has established contrast ratios limits to satisfy sunlight readability criteria. These criteria are published in, "Night Vision Goggle Lighting Specification." The specifications are shown below: C1--OnLED to Background C2--OnLED to OffLED C3--OffLED to Background Optical Filtering An LED, regardless of its brightness, has a difficult time competing with the sterance of the sun. An LED display's readability can be greatly improved by using contrast enhancement filters. The filter of choice is one that eliminates the interference of the sun with the background of the display. A number of filter vendors offer anti-reflection coated, circular polarized (AR/CP), optically tinted bandpass and neutral density filters which have proven very helpful in satisfying the sunlight readability contrast requirements. The display front surface and the areas surrounding the LEDs have specular reflector characteristics. This reflective property allows optimum contrast when used with a circular polarized filter with anti-reflective coating. See Table 1 for Filter Selection Guide. 2.0:1 Minimum 2.0:1 Minimum 0.25:1 Maximum
2000 Infineon Technologies Corp. * Optoelectronics Division * San Jose, CA www.infineon.com/opto * 1-888-Infineon (1-888-463-4636) OSRAM Opto Semiconductors GmbH & Co. OHG * Regensburg, Germany www.osram-os.com * +49-941-202-7178
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Table 1. Contrast enhancement filter selection guide Display P/N or Condition ISD2010 ISD2012 ISD2352* ISD2310 ISD2312 ISD2011 ISD2351* ISD2311 ISD2013 ISD2353* ISD2313 High Ambient Light High Ambient Light ISD2010 ISD2012 ISD2352* ISD2310 ISD2312 ISD2011 ISD2351* ISD2311 ISD2013 ISD2353* ISD2313 High Ambient Light Display Color Red/ Hi Eff. Red Transmission 25% at 635 nm Filter Color Red Filter Model No. Filter Manufacturer MPC 20-15C Marks
Yellow
25% at 585 nm
Amber
MPC 30-25C Marks MPC 50-22C Marks MPC 80-10C Marks MPC 80-37C Marks HLF-608-5R Hoya
Hi Eff. Green
22% at 565 nm
Yellow/Green
All Colors All Colors Red/ Hi Eff. Red
10% Neutral 37% Neutral 14%
Neutral Gray Neutral Gray Reddish Orange
Yellow
14%
Yellowish Orange Yellow/Green
HLF-608-3Y Hoya HLF-608-1G Hoya HNCP10 Polaroid
Hi Eff. Green
14%
All Colors
10% Neutral
Neutral Gray
* Sunlight viewable displays. All other part numbers represent all the standard Military Small Alphanumeric Displays.
Manufacturers Marks Polarized Corporation 25B Jefryn Blvd. West Deer Park, NY 1179-5715 (516) 242-1300 HOYA Optics, Inc. 3400 Edison Way Fremont, CA 94538-6138 (415) 490-1880 Polaroid Corp. Polarizer Division 1Upland Road Norwood, MA 02062 (617) 577-2000
2000 Infineon Technologies Corp. * Optoelectronics Division * San Jose, CA www.infineon.com/opto * 1-888-Infineon (1-888-463-4636) OSRAM Opto Semiconductors GmbH & Co. OHG * Regensburg, Germany www.osram-os.com * +49-941-202-7178
Appnote 47
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Contrast Measurements The ability to read a display in direct sunlight used to be determined by using a series of standard observers and irradiating the display with one sun. Recent technical studies have established contrast criteria permitting laboratory measurements that verify sunlight readability. A yellow ISD2351 and a high efficiency green ISD2353 Small Alphanumeric Display were evaluated for sunlight readability under a simulated sun with an incident of 4200 fc. These two displays were tested with both pass-band and neutral density AR/CP filters. The measurement technique and results follow. Figure 1. Sunlight readability contrast measurement setup
Contrast Measurement Setup Contrast measurements require the use of a spot photometer, which measures the luminance of the surface within a specific spot size. A Photo Research Spectra Pritchard spot photometer Model 1980B with a Macro Spectar MS-80 Lens was calibrated and set to read out in foot Lamberts, fL. The display and filter were mounted on a micro adjustable X/Y/Z stage. This stage, the spot photometer, and a 500 W Unimat LX80 light source were mounted on a optical bench. The light source was oriented 30 from the normal of the display. See Figure 1.
2000 Infineon Technologies Corp. * Optoelectronics Division * San Jose, CA www.infineon.com/opto * 1-888-Infineon (1-888-463-4636) OSRAM Opto Semiconductors GmbH & Co. OHG * Regensburg, Germany www.osram-os.com * +49-941-202-7178
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Figure 2. LEDs selected for measurement
Figure 3. Points for luminous stearance measurement
See Figure 3 for exploded view
La Lb
Ld
Lc
The sterance measurements were made using the photometer with an angle setting of 2 minutes resulting in a spot size of .004 in. The incidence flux was determined by using a Kodak 6080 standard lambertian reflector painted slide. The X/Y/Z stage supported the display, the AR/CP filter, and the display drive electronics. Figure 3 shows the asterisk (*) programmed on the display. Each LED had a duty factor of ON 17.6%. The center of the asterisk was used as the measurement LED. The stage was used to position the display at the four contrast measurement points as shown in Figure 2. Contrast Calculations The data derived from the spot photometer was used to calculate the three contrast ratios, C1, C2, C3. For best accuracy, LB was the average of three spot locations. Figure 3 shows these as La, Lb, Lc. La is the substrate sterance between two LED die, Lb is the substrate sterance of the area between four LED die; and Lc is the sterance of a gold trace connecting the LEDs.
Measurements were made using a yellow ISD2351 and a Marks MPC80-10C neutral density gray filter. This display had a typical intensity of 2450 cd/LED with an average wavelength of 585 nm. Measurements were also made using a green ISD2353 with a Marks MPC50-22C yellow/green bandpass filter and a Marks MPC80-10C neutral density gray filter. This display had a typical intensity of 3470 cd/LED with an average wavelength of 572 nm. The data and results of the experiment are shown in Table 2. Conclusion From the data, the most readable combination is the green ISD2353 display and a green bandpass AR/CP filter, followed by the green display and the 10% transmissive neutral density gray AR/CP filter. In both cases these combinations exceeded the military limit by almost 2.5 times. The yellow display has optimum contrast with a neutral density AR/CP gray filter.
Table 2. Luminous contrast at 4200 fc Display Color Filter Model # Status La Green MPC50-22C Green MPC80-10C Green MPC50-22C Yellow MPC80-10C Yellow No Filter LED-On LED-Off LED-On LED-Off LED-On LED-Off LED-On LED-Off LED-On LED-Off 12.00 11.00 9.70 8.80 12.60 11.10 9.30 7.80 208.00 198.00 Lb 10.20 9.40 8.60 8.60 11.30 9.80 8.20 7.20 198.00 171.00 Lc 27.10 20.60 17.80 11.40 37.30 17.70 19.40 10.30 650.00 853.00 Ld 101.60 12.60 69.80 10.10 111.00 14.40 46.50 7.70 480.00 171.00 Footlamberts LB 16.43 13.67 12.03 9.60 15.30 12.87 12.30 8.43 352.00 407.33 C1 5.18 4.80 6.25 2.78 0.36 C2 7.06 5.91 6.71 5.04 1.81 C3 0.08 0.05 0.12 0.09 0.58
2000 Infineon Technologies Corp. * Optoelectronics Division * San Jose, CA www.infineon.com/opto * 1-888-Infineon (1-888-463-4636) OSRAM Opto Semiconductors GmbH & Co. OHG * Regensburg, Germany www.osram-os.com * +49-941-202-7178
Appnote 47
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