My dishwasher shines a red light onto the floor whilst washing. At the end, this turns off.
Is there a smart device that is ideal to recognise the light turning off by being positionedon the floor beneath it (or possibly just to the side if it can be angled easily)? I know there are light sensors but these seem to be best suited to recognise daylight.
Any suggestions?
If you are in the US or Canada, Homeseer has a Z wave device made for exactly that purpose. 
I know some community members have been using it, but I don’t know if it works with the new V3 app yet.
However, the “whilst“ in your post concerns me that you might be in the UK and not have mentioned it. since device selection does vary by region, the country you are in is important information. 
Homeseer does not make a version that works on the UK zwave frequency, unfortunately.
If you are in Europe or the UK, You could probably find a similar dumb sensor at a robotic site and hook it up to an Arduino, but that would be a lot of work.
Something like this, for example:
If you are interested in that type of approach, @ogiewon should have some ideas. But again, that device in itself doesn’t have a radio, it’s just a sensor that you add to something else before you can get integration with smartthings.
For clarification, I am in the UK.
why not use a smartthings open/close acceleration multisensor with the dryer app. when the dryer (dishwasher) stops vibrating it will tell you its done.
Is the light recognition a hard requirement? Could you use a smart plug and automate when usage drops below a given level (ie when the dishwasher is finished) to alert you?