Abstract: A frost sensor for detecting the presence of frost in a refrigeration device includes a light impervious enclosure having an air passage through it. Means preferably are provided for directing ambient air through the air passage. The air passage has thermal conducting means mounted in it, the thermal conducting means having a first surface forming a portion of the air passage. A connection device also is provided for attaching the conducting means to the refrigeration device. A photocell and a light source are mounted so that the photocell senses reflectivity of the conductive means. The volume of ambient air passing through the air passage and the thermal resistance between the conducting means and the refrigeration device may be varied so that the formation of frost on the conducting means can be made to agree with the formation of frost on the refrigeration device.