Called 'Candling box', farmers can use the kit to know which hens lay eggs that cannot be hatched and are due for culling, according to Business Daily of Kenya.

According to Geoffrey Kago, the inventor, he is selling about 50 of the candlers a month compared to 10 this time last year.

He came up with the concept after he encountered the challenges in his own poultry farming in selecting between eggs to put into the incubator and those to leave out.

The candler helps "assess the defects in an egg and check its progress, like nutrition", explained Mr Kago.

The candler is rectangular in shape and slightly bigger than a brick and has a hole on one side designed to fit the pointed side of the egg. Inside, it is hollow with a lighting fixture. To check the egg's condition, the farmer places its pointed side in the hole and lights the candler in a dark room.

If the farmer notices the air sac in the less pointed side is sagging and big, it is a sign the egg has stayed for too long and thus dehydrated. Therefore, it cannot be placed in an incubator to hatch, since it has lost 15 per cent of the water in it.

Ideally, according to Mr Kago, to select an egg for hatching it must be less than seven days old.

"As the egg ages, the air space becomes bigger," he says. If the farmer sees two yolks in an egg, it is also unsuitable for incubation.

Mr Kago also advises farmers to check the outward physical appearance of eggs and ensure that they are of uniform colour and size for that particular bird breed.

Eggs with blotches, wrinkles, bumps or multiple colours may not hatch if incubated, as it is a sign of nutrient deficiency in the hen that laid the eggs.

Besides checking the egg before it is placed in the incubator, the candler helps farmers to gauge the progress of eggs waiting to hatch in the incubator. Thus, a farmer is able to isolate spoilt eggs and tell the number of chicks one is likely to get, reports Business Day.