? 2000 in?eon technologies corp. ?optoelectronics division ?san jose, ca www.in?eon.com/opto ?1-888-in?eon (1-888-463-4636) osram opto semiconductors gmbh & co. ohg ?regensburg, germany www.osram-os.com ?+49-941-202-7178 1 may 31, 2000-14 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 ef?iency of leds. polarized ?ters enhance the readability of leds by eliminating much of the ambi- ent re?ction from the led die and the surrounding package. readability criteria an observer's ability to perceive the information from an alpha- numeric 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 envi- ronment. high readability results by optimizing the following contrast ratios. the ?st ratio, c 1 , involves the onled stear- ance, l ledon , and the background stearance, l b , is optimized when the onled is brighter than the surrounding area. thus c 1 will be much greater than unity (1). the next ratio, c 2 , involves the sterance ratio of the on, l ledon , and off, l ledoff , led. c 2 is optimized when the l ledon is much greater than the l ledoff , 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. 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 speci?ation. ?the speci?ations are shown below: c1?nled to background 2.0:1 minimum c2?nled to offled 2.0:1 minimum c3?ffled to background 0.25:1 maximum optical filtering an led, regardless of its brightness, has a dif?ult time com- peting with the sterance of the sun. an led display s readabil- ity can be greatly improved by using contrast enhancement ?ters. the ?ter of choice is one that eliminates the interfer- ence of the sun with the background of the display. a number of ?ter vendors offer anti-re?ction coated, circular polarized (ar/cp), optically tinted bandpass and neutral density ?ters which have proven very helpful in satisfying the sunlight read- ability contrast requirements. the display front surface and the areas surrounding the leds have specular re?ctor characteristics. this re?ctive property allows optimum contrast when used with a circular polarized ?- ter with anti-re?ctive coating. see table 1 for filter selection guide. sunlight readability contrast measurements for the ISD2351 and isd2353 serial input small alphanumeric display appnote 47 by bob krause ratio equations 1) c 1 ?nled to background 2) c 2 ?nled to offled 3) c 3 ?ffled to background c 1 l ledon l b l b ------------------------------- = c 2 l ledon l ledoff l ledoff -------------------------------------------- = c 3 l ledoff l b l b --------------------------------------- =
? 2000 in?eon technologies corp. ?optoelectronics division ?san jose, ca appnote 47 www.in?eon.com/opto ?1-888-in?eon (1-888-463-4636) osram opto semiconductors gmbh & co. ohg ?regensburg, germany www.osram-os.com ?+49-941-202-7178 2 may 31, 2000-14 table 1. contrast enhancement ?ter selection guide * sunlight viewable displays. all other part numbers represent all the standard military small alphanumeric displays. manufacturers marks polarized corporation hoya optics, inc. polaroid corp. 25b jefryn blvd. west 3400 edison way polarizer division deer park, ny 1179-5715 fremont, ca 94538-6138 1upland road (516) 242-1300 (415) 490-1880 norwood, ma 02062 (617) 577-2000 display p/n or condition display color transmission filter color filter model no. filter manufacturer isd2010 isd2012 isd2352* isd2310 isd2312 red/ hi eff. red 25% at 635 nm red mpc 20-15c marks isd2011 ISD2351* isd2311 yellow 25% at 585 nm amber mpc 30-25c marks isd2013 isd2353* isd2313 hi eff. green 22% at 565 nm yellow/green mpc 50-22c marks high ambient light all colors 10% neutral neutral gray mpc 80-10c marks high ambient light all colors 37% neutral neutral gray mpc 80-37c marks isd2010 isd2012 isd2352* isd2310 isd2312 red/ hi eff. red 14% reddish orange hlf-608-5r hoya isd2011 ISD2351* isd2311 yellow 14% yellowish orange hlf-608-3y hoya isd2013 isd2353* isd2313 hi eff. green 14% yellow/green hlf-608-1g hoya high ambient light all colors 10% neutral neutral gray hncp10 polaroid
? 2000 in?eon technologies corp. ?optoelectronics division ?san jose, ca appnote 47 www.in?eon.com/opto ?1-888-in?eon (1-888-463-4636) osram opto semiconductors gmbh & co. ohg ?regensburg, germany www.osram-os.com ?+49-941-202-7178 3 may 31, 2000-14 contrast measurements the ability to read a display in direct sunlight used to be deter- mined by using a series of standard observers and irradiating the display with one sun. recent technical studies have estab- lished contrast criteria permitting laboratory measurements that verify sunlight readability. a yellow ISD2351 and a high ef?iency 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 ?ters. 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 speci? 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 ?ter 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 ori- ented 30 from the normal of the display. see figure 1.
? 2000 in?eon technologies corp. ?optoelectronics division ?san jose, ca appnote 47 www.in?eon.com/opto ?1-888-in?eon (1-888-463-4636) osram opto semiconductors gmbh & co. ohg ?regensburg, germany www.osram-os.com ?+49-941-202-7178 4 may 31, 2000-14 figure 2. leds selected for measurement 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 ?x was determined by using a kodak 6080 standard lambertian re?ctor painted slide. the x/y/z stage supported the display, the ar/cp ?ter, and the display drive electronics. figure 3 shows the asterisk (*) pro- grammed on the display. each led had a duty factor of on 17.6%. the center of the asterisk was used as the measure- ment 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 calcu- late the three contrast ratios, c 1 , c 2 , c 3 . for best accuracy, l b was the average of three spot locations. figure 3 shows these as l a , l b , l c . l a is the substrate sterance between two led die, l b is the substrate sterance of the area between four led die; and l c is the sterance of a gold trace connecting the leds. see figure 3 for exploded view figure 3. points for luminous stearance measurement measurements were made using a yellow ISD2351 and a marks mpc80-10c neutral density gray ?ter. this display had a typical intensity of 2450 ?d/led with an average wavelength of 585 nm. measurements were also made using a green isd2353 with a marks mpc50-22c yellow/green bandpass ?ter and a marks mpc80-10c neutral density gray ?ter. this display had a typi- cal intensity of 3470 ?d/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 ?ter, followed by the green display and the 10% transmissive neutral density gray ar/cp ?ter. 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 ?ter. lc ld la lb table 2. luminous contrast at 4200 fc display color filter model # status footlamberts l a l b l c l d l b c 1 c 2 c 3 green mpc50-22c led-on led-off 12.00 11.00 10.20 9.40 27.10 20.60 101.60 12.60 16.43 13.67 5.18 7.06 0.08 green mpc80-10c led-on led-off 9.70 8.80 8.60 8.60 17.80 11.40 69.80 10.10 12.03 9.60 4.80 5.91 0.05 green mpc50-22c led-on led-off 12.60 11.10 11.30 9.80 37.30 17.70 111.00 14.40 15.30 12.87 6.25 6.71 0.12 yellow mpc80-10c led-on led-off 9.30 7.80 8.20 7.20 19.40 10.30 46.50 7.70 12.30 8.43 2.78 5.04 0.09 yellow no filter led-on led-off 208.00 198.00 198.00 171.00 650.00 853.00 480.00 171.00 352.00 407.33 0.36 1.81 0.58
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