The condenser provides even illumination to the negative beneath it. Condenser enlargers produce higher contrast than diffusers because light is scattered from its path by the negative's image silver; this is called the Callier effect. The condenser's increased contrast emphasises any negative defects, such as dirt and scratches, and image grain. A point source enlarger is a variation of the condenser enlarger designed to cut light diffusion above the negative. Contrast is enhanced and grain in the resultant print is sharper than with a conventional enlarger, and the transition from light to dark at the edge of the shadow areas is dramatic. An unfrosted clear lamp with a tiny filament is used without diffusers. As the illuminant is narrow the lamp must be precisely positioned both vertically and horizontally, because the condensers project only that single small filament rather than light that fills the whole housing. However the lens must be kept at full aperture to avoid projecting an image of the light source restricted to the centre of the baseboard, which will cause vignetting and falloff in the print. Exposure is controlled through duration or using a variable transformer. A diffuser enlarger's light source is diffused by translucent glass or plastic, providing even illumination for the film. Diffuser enlargers produce an image of the same contrast as a contact print from the negative. Cold light or cold cathode enlargers employ diffusion enlarger heads with a coiled Fluorescent lamp tube rather than a conventional light bulb. Their light is blue-rich, in an area of the spectrum to which silver gelatin paper is sensitive, and therefore exposure is shorter comparative to that with other light sources, ideal for making large mural prints which require extended exposure, and heat is reduced which is beneficial in avoiding buckling or 'popping' of negatives, and also are Newton's rings' where a glass negative carrier is used. They produce a softer (less contrasty) print. Color enlargers typically contain an adjustable filter mechanism - the color head - between the light source and the negative, enabling the user to adjust the amount of cyan, magenta and yellow light reaching the negative to control color balance. Other models have a drawer where cut filters can be inserted into the light path, synthesize colour by additive mixing of light from colored lamps with adjustable intensity or duty cycle, or expose the receiving medium sequentially using red, green and blue