Halation occurs when light from a bright object passes through the sensitive layer of the film and is reflected back onto the sensitive film by the rear of the glass plate or other base, causing a faint halo around the image. Different measures were taken to overcome the problem:
- The sensitive layer could be dyed to prevent light reaching the base.1
- The back of the plate could be coated with a light-absorbent layer. The backing was dissolved when the plate was developed.
- A light-absorbent layer could be placed between the sensitive layer and the base.
- The plate could be coated with multiple layers of emulsion having different sensitivity. The slowest, and most opaque, was next to the glass support. Plates of this type were introduced by J.T. Sandell in 1891 and sold by R.W. Thomas the following year.2
- The back of the plate could be finely ground so preventing a strong reflection. This type of plate was sold by Samuel Fry, R.W. Thomas and others in the 1890s.3
There was much discussion in the wet collodion period as to the cause of halation and what to do about it, some photographers had the idea that it was due to the lens rather than reflection from the glass plate. The common methods of overcoming the problem used at that time were to either coat the back of the plate or introduce a dye into the collodion. In the dry-plate era plates started to be produced with an anti-halation backing applied by the manufacturer, though some photographers still applied their own backing.
References & Notes
Clerc, Photography. Theory and Practice, p. 159. Cyclopedia of Photography, pp. 32, 367. PA 1891, p. 473.
 John Spiller provided a list of colouring matter that could be included in emulsion to prevent halation. BJP 17/4/1874, p. 182.
 BP 2183/1891. BJA 1893, p. 887. BJA 1894, p. 859. Sandell films on a celluloid base were also produced.
 Fry advertisement BJA 1890, p. 206. PA 1891, p. 473. Thomas advertisement PA 1891, p. 474.