Techtalk17


	Technical Talk Part 17
		Testing a darkroom safelight.
	This is a simple test to check how safe a darkroom safelight is (no safelight is totally 'safe'). This test is both for B&W & colour papers. For colour set the filters to give a roughly white test. For multigrade, set them for the lowest grade possible. In complete darkness & with no negative in the carrier of the enlarger make & process a test strip in order to determine the exposure required to produce just a slight density (fog) on the paper. This is because paper that has been exposed is more sensitive than unexposed paper, so we need to mimic exposing a print. You will need to do this test for each paper you use, since speeds & safelight sensitivity do vary. Then, still in complete darkness, tape a coin in the corner of another piece of paper & expose it using the fogging exposure found with the test strip. Lay the exposed paper, still with the coin taped on it, on the bench or enlarger baseboard, whichever is nearest to the safelight. Place four coins on the paper, one in the corner next to the taped coin, & turn on the safelight. Wait for 2 minutes then remove the coin farthest from the taped coin. Wait another 2 minutes then remove the middle coin, then another 2 minutes and remove the nearest coin. Don't remove the forth coin in the corner. After waiting another 2 minutes, turn off the safelight, remove the last two coins & process the paper in complete darkness. Depending on how safe the safelight is & your fogging exposure, you will have a piece of paper something like this: This example shows that, compared with the corner fogged but unexposed to the safelight the 2 minute exposure has produced no noticeable increase in tone. However, the 4 minute exposure has. This indicates that the safelight is only 'safe' for about 2 minutes before it will begin to darken the very lightest highlights in a print. Now in practice, not only does the distance from the safelight of a piece of paper vary as it moves from box, to enlarger & into the developer dish, but once it's in the developer both the brown colour & a desensitising effect help to protect the paper a little. You can repeat this test with the safelight on (you don't need to tape on the coin). Then cut the processed test in half along the length of the coins so that it can laid on top of the first to see if there is more fogging. If the result of the test shows that the safelight doesn't give you enough 'safe' time to expose & process prints, there are quite a few things that you can do to improve it. You can move the safelight further away, fit a lower wattage bulb if possible, fit an electronic dimmer or bounce the light off of a wall or ceiling. (Warning. Be very careful that anything you place the safelight near will not get hot enough to fume or be a fire risk. Always follow the manufacturer's instructions about safe distances. Always consult a qualified electrician to ensure safety.) You can also turn off the safelight when the enlarger lamp is on. Never leave paper, whether exposed or unexposed, out under the safelight, keep it covered when not actually being used. This not only reduces the cumulative effect of exposure, but also prevents fogging by light spill from the enlarger & by the room white light being turned on accidentally. Finally, the filters in safelights can degrade as they get older, so it's a good idea to do a safelight check every now & then to make sure. © Barry Leighton FRPS