Light Stress Explanation:

Light Burn happens when your grow lights are either too close to the tops of your plants, or they are putting out too much heat onto the plants.  Either way, light burn will affect the top leaves first since they are the ones closest, and will cause them to yellow.  As it progresses, the tips of the leaves will brown and begin to curl up while turning crunchy.  People often confuse Light Burn with Nitrogen Deficiency and vice versa.  But the easiest way to differentiate the two is by where the leaves yellow first.  If they yellow up top first, it's Light Burn.  And if they yellow down low first, it's Nitrogen Deficiency.

Early Stage Light Burn

Late Stage Light Burn

How to Cure:

All you need to do to fix this issue is raise your grow lights.  Every type of grow light and strength requires a different distance from the tops of your plants, so always read what the instructions that came with the light recommend.  If all else fails, it is best to raise the lights as high as possible and slowly lower the lights until it seems like the plants start showing signs of light burn.  Then you know what the minimum distance should be!  It helps to have adjustable light hangers for this as well.

More Examples of Light Stress:

Tallest leaves slightly yellowed while lower leaves are green.

Leaf Fingers show stripes of yellow and lime green.