Wanhao Duplicator 7 Mask Creation Toolset

One of the issues people are encountering when printing using an LCD-based 3D printer, such as Wanhao Duplicator 7, is the need for a mask, if the entire build plate area is to be used. This is because no matter of light source is used, there are differences in light intensity across the printing area. If these are not compensated for, there may be issues with the print, like over or under exposure, or worse the print may fail at all.
For compensation a mask is to be used. A mask is an image, of the size of the LCD screen, that will be darker in the more illuminated areas and lighter in the less illuminated area. Ideally, if only this image will be displayed on the LCD, the result will be that each pixel will be let the same amount of light to pass through, thus resulting in an uniform illumination of the resin.
Obtaining a mask is not a trivial think though. Basically, two different methods where devised by communities around different 3D printer models.
One is based on taking a picture of the LCD through a diffusing screen (such as a piece of paper), and graphically processing the image to basically "invert" the image.
The other will measure the light across the entire LCD screen, using a sensor, then generate an image that will do the job. This may sound a bit magically and too technically perhaps for most users. Therefore, any implementation of the method should be simply enough and use, if possible, readily available software applications that one may have already installed or at least can be easily installed and used.
The toolset proposed here is composed of two parts.
The hardware part is a made of parts printed using a FDM/FFF 3D printer. The actual measurement part is a light sensor, that could be basically anything that can transform light into something electrically measurable. My installation uses a photoresistor which resistance value is measured by a regular multimeter instrument.
The software part is based on a Microsoft Excel spreadsheet where the measured values are entered and the "brut" mask is generated as an array of grey-shaded cells. This is further processed into a smooth grey-gradient image that is the actual mask.
The thing as is can be used for Wanhao Duplicator 7, but one may get inspired and adapt it to other LCD-based (or even projector-based) 3D printers.

Update history
04.03.2018

• thing created, with summary
• stl models for hardware part uploaded