> Just received an RFP (request for proposal) for a semi-custom terminal > from a subcontractor for the US Army. The specs are entirely metric, > with one exception: character height on the display is specified > at 0.15". All other specs, including mechanical, keypad sizes, > temperatures, sealing pressures, etc. are metric. > > I can't explain the 0.15" (3.81 mm) -- it is not any kind of standard for > character displays. The ubiquitous 4x20 LCD modules used in these devices > nearly universally use characters that are 4.75 mm high. > > I believe the specification is from the Army, and not a metric rendition > created by the subcontractor.
I should be able to provide you with the answer since it is my field. However, I must admit that I am baffled too. The standard that I would expect to see for this application would be MIL-STD1472. It has traditionally dealt with painted labels and the version 'F' of the standard gives a range of either 2.5-5 mm or 3-5 mm. https://www.denix.osd.mil/denix/Public/Library/Ergonomics/milstd-1472.pd f A previous version 'D' (which still may be on many bookshelves) gave exact values of 2.3 mm and 4.7 mm. http://jcs.mil/htdocs/teinfo/directives/soft/ms1472d.4.html However, self-illuminated characters (as on an LCD) do not appear to be addressed by that standard and I cannot put my hands on anything immediately. Something that springs to mind immediately is that '15 minutes of arc' is a well known (in the human factors community) size for characters. Perhaps somebody has done a calculation based on viewing distance and worked out a character size using imperial measures. If you get a round number in imperial (26 inches is sometimes used) then it will be a plausible scenario. I am too lazy to work out the viewing distance for 3.81 mm characters and that angular subtense. There are also some plausible error scenarios where 15' becomes 0.15" It is also possible that you have been given a legacy size which replicates current equipment. However I would want to see what the standards say for self-illuminated characters before sticking my neck out. The US Army should certainly be using metric measures so there is definitely something odd. However, oddities like this no longer surprise me. They provide me with food. -- Terry Simpson Human Factors Consultant [EMAIL PROTECTED] www.connected-systems.com Phone: +44 7850 511794
