In my application the user needs to read a few simple shapes and characters under the bright sunlight. In terms of visibility, what would be the ideal solution? I am basically looking for experience of previous implementors or some research on the subject. Currently considering simple LED matrix (8×8) and blue color.
Electronic – LED matrix display – outdoor
led-matrix
Related Solutions
You won't find a chip that can drive 128 LEDs but there are a few that can drive 16 columns: -
There is another TI chip that can do 24 channels of LEDs - the TL5947 and there's also a 32 channel chip from linear tech called the LT3746 - it has integrated power management to suit multiple leds in series and varying supply voltages.
Since you are using so many LED’s, I will assume you are using inexpensive common LED’s. Typical LED needs about 1.5 volts across LED (look up your LED voltage at the current you choose for your LED). Depending upon the brightness you desire, you could end up somewhere between 10mA and 20mA.
Using 74HC595, looks like you will be driving the LED’s using 5 volts out. So, if LED voltage is 1.5 volts, then the resistor voltage will be 5v – 1.5v = 3.5v . . Use ohms law to calculate resistor R = V/I .
One word of caution. The 74HC595 on several different data sheets shows a maximum of 6mA output drive or current sink capability. Data sheet also shows output clamp current of 20mA, depending upon the manufacturer. If you try to get the 74HC595 to drive more than 6mA, the output voltage of the 74HC595 will begin to depart from being 5 volts.
You may want to set up one of your 74HC595 shift registers with a resistor and LED on the output to assure the performance is as you desire. In your schematic the 74HC595 will be sourcing current, not sinking current, but the data sheet seems symmetric for output current sinking or sourcing.
As you draw more and more current from your 74HC595, the output voltage will begin to droop (reduce) down from 5 volts. Also, the 74HC595 will begin to heat up some more.
EDIT : Just noticed that if you turn on ONE Column, and MORE than one Row, you will be lighting more than one LED. This puts the several LED's in parallel, and the brightness of each of those LED's will dim. Fortunately the resistor will still limit the current drawn from the 74HC595.
EDIT EDIT : Also just noted you did include your LED as 3.8 volts at 0.02 (20mA). Your resistor is going to end up near 100 ohms. That is a lot of current to ask from the 74HC595.
Best Answer
Use some good old Histogram Equalization to reduce glare effects, reduce shadow effects, and bring out colours better. Then process the image for "Chromaticity" - the equation is very simple and is seen on this Wiki page with an explanation.
If you then examine the blue colour values only of the RGB image, you will see it very well indeed.
I have done this kind of colour space processing before in robotics, and even tutored a unit relating to this (coloured targets for robot arm movement). I did not actually do Histo-equalization though, but in outdoor environments give it a shot.
I suggest if you have access to MATLAB that you check out my professor's MATLAB Toolbox, by Peter Corke (very well known guy in robotic vision and control. check out his textbook for direct use with this toolbox and examples )
His toolbox allows really quick image processing and will at least give you some decent examples for your own work. Get the toolbox here