If it's bright enough so that you can just barely read a newspaper then it's enough to stimulate egg production. It has little to do with the birds' eyes. The light is absorbed by the hens' bodies and stimulates production of hormones that trigger reproduction. Even a blind hen will respond to changes in the amount of daylight.
I was reading on this board in another post about heat lamps. The poster said they cost about $30 a month to run and you have to be very careful so nothing catches on fire because of them. Wonder how much the light in your picture would cost to run each month just for extra eggs? Would it be worth it? I only have 7 hens, so probably not. We had quite a few chickens 19 years ago here in Maryland and never heated their barn. Now I'm hearing about people wanting to heat the coops. We certainly didn't have nice forums on an internet to find out all the information we learn now. I love seeing all the beautiful pictures of everyone's chickens. Makes me want one of each!!