exposure units

Polymer plates are sensitive to UV light. The sun could do the trick, but it is not very accurate. Clouds, time of the year, hour of the day..... all will interfere with your exposure time.
So you will need access to a UV light unit.  There are different lighting units, you can buy one or build one. If you have access to different units, and you plan on using more than one, you have to test them all. If you are interested in more technical know how, read this article on the 'unblinking eye' website.

1.HPR lam;p;
The Philips HPR 125W bulb (HPR125 Mercury Discharge Reprographic Source). A lot of screen printers used this source. It delivers a powerful bluish-white light with strong actinic radiation. To operate it you need a transformer for each bulb.before exposure, the lamps needs to be on full power, this will take a couple off minutes. If you switch them off, let them cool down before switching on again

- How to connect a HPR bulb to a Philips BHL 125 L40 ballast.

2.Metal halide exposure units;
I use this one for the moment. The older ones need to heat up and completely cool down if switched off. Heating takes a few minutes.You can find second hand metal halide vacuum exposure units from a printing office. Check the internet for a decent priced one. Replacement bulbs are rather expensive.

3.UV fluorescent exposure units;

This one is easy to make if you like a little DIY.  Make sure to use the right UV fluorescent tubes. (e.g. Sylvania F25 T8 BL350 18)

These units produce less heat and you don't need a lot of distance between your unit and your plates. So it could be a table top unit.
DIY links:
http://www.atm-workshop.com/light-box.html
https://www.youtube.com/watch?v=_Zif0NBOeBc
https://www.youtube.com/watch?v=p-q2lkAMbdA

      Sylvania F25 T8 BL350 18. Click to enhance.
     

4. UV LED exposure units;
Low one energy consumption, and the life time of an LED bulb is between 50.000 and 100.000 hours. There is no waiting time, switch on the unit and it's ready to work.
A bit harder to build yourself, but you could.
DIY links:
https://www.youtube.com/watch?v=_2ZlrqMdTm8




DIY a few more;
http://www.screenprinting-aspa.com/how-to-articles/how-to-build-an-exposure-unit
https://www.pinterest.com/pin/184436547216133011
https://www.youtube.com/watch?v=D9IyQVFosFQ

You can find a lot of information on the net. Just find the one you would fancy. Print offices, screen printers, polymer plates... All of these units will work for you.

Protect yourself from UV light, and don't look into the UV light. UV exposure is not immediately felt . . . the user may not realize a hazard until after the damage is done.