Introduction: Measuring the Pixel Resolution of LCD Screens
The need to measure pixel resolution arose from buying some 5” car monitor LCD screens that were specified as 800x480 pixel resolution and realising I had been supplied with 480x272 pixel versions. The picture on the lower resolution screens is not quite as sharp. These screens take a PAL/NTSC composite input and hence the resolution is not so easy to check. I expect most customers do not notice and hence sellers can get away with supplying lower specification products. This seems to be a widespread problem that is getting worse. I have had problems with suppliers on both ebay and AliExpress. Hence I thought it useful to show that it is not particularly difficult to measure the pixel spacing – just using a camera and a ruler – something the last seller said was not possible. So here is the simple art of the impossible!
I hope this is helpful to both buyers and sellers of LCD screens. My last supplier did not seem to have any means of checking their products – so it could be their source that was the problem. Trying to get this right can save a load of hassle. I ordered 11 off (from a supplier who had supplied correct items a year earlier!) and they were all wrong. AliExpress agreed eventually they were not as described – but only offered 30% refund plus no refund of the duty I had paid. So much for their buyer protection and refund if not as described – so beware! It is best to get one off first to check if you want to order a greater number. In all cases I had contacted the seller to emphasise my requirement for 800x480 pixel resolution. For what it’s worth my application was to show a continuous video from a Raspberry Pi zero – using the composite video signal. The space available did not have space for the connectors on HDMI screens. I had agreed the higher spec. screens with the guy I was making the systems for and I want my work to look as good as it can.
- Take photo(s)
- Zoom in and count the pixels over a few mm
- Camera - mine was an old 10megapixel Panasonic Lumix – with a manual focus option
- Ruler – I used a steel ruler with 0.5mm markings but 1mm marks would have been fine
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Step 1: Take Photo(s)
This does require a little care and a few shots to get one with a good sharp picture.
First set up the screen with an input that has at least sections that are predominately red, green or blue – the pixel colours. On the Pi I just used the video as this has some areas of blue. I have in the past used a program to show a bmp file full screen - see https://www.raspberrypi-spy.co.uk/2017/02/how-to-display-images-on-raspbian-command-line-with-fbi/. If using a camera input, just point the camera at a plain card that is mostly one of the pixel colours.
Now to the camera. Most will be OK for this job. Manual focus probably helps. A quick note on resolution. My 10megapixel camera gives pictures with a resolution of 3648 x 2736. So a picture of the full monitor screen gives more than 4 picture pixels for every screen pixel at 800 pixels horizontal and more than 7 picture pixels for every screen pixel at 480 pixels horizontal resolution. This is enough to pick out individual pixels and hence count the number of pixels over a set distance. I used 5mm.
I decided to take a picture of the full screen -so the supplier could see it was theirs. If your camera has less resolution you may need to cover a smaller section.
My trick with focussing was to use the manual option. I found that when the focus was right that Moire fringes would appear. Macro autofocus should work but will focus on the ruler rather than the screen. Be prepared to experiment a bit if you don’t get the sharpness required.
Step 2: Zoom in and Count the Pixels Over a Few Mm
By this I mean zoom into the picture to see if the pixels have been captured and then do a count over a short distance. I opened the pictures in Windows Paint but presume other systems will have something just a good.
When I first loaded up my picture to AliExpress they did not understand that they had to zoom in – and so dismissed my dispute. It is easy to forget to explain what seems obvious – but may not be to all! Also their system may not have allowed zooming into pictures. This is what I uploaded:
So I then sent them a second picture – that just had a zoomed in section of the first. In this the individual pixels were visible. I then added the pixel count and calculation to show the number of pixels expected over 5mm for a screen width of 108mm and resolutions of 800 or 480. This was much clearer and they agreed. Second picture:
I hope this helps anyone caught in the same trap get a refund if the supplier is not able to provide the product they paid for. Suppliers can also use this technique to argue with their sources. Hopefully the outcome will be less deception and more happy customers.