Duct grease deposit detection devices, systems, and methods

A method for detecting fouling in a duct, comprising:

• placing a member in an exhaust duct of an kitchen exhaust hood with a surface in an exhaust stream flowing in said duct;
• the kitchen exhaust hood receiving fumes containing filterable particles of flammable grease;
• the kitchen exhaust hood having a particulate filter at a position upstream of said member and effective to filter out said filterable particles;
• and
• generating an output signal indicating a fouled condition of the surface due to a change in a property of the surface indicative of fouling;
• wherein the output signal is generated by a detector that includes a light source and a light detector, the member having a planar surface facing the light source and the light detector, the member being oriented and positioned to reflect light from said light source to said light detector, the member being positioned inside the duct;
• wherein said light source and said light detector being positioned adjacent each other and opposite said member planar surface;
• wherein said duct having a generally planar wall, said light source and said light detector being on an opposite side of said planar wall from an interior of said duct and said member surface being positioned on n interior side of said planar wall such light detector and the light source are located together opposite the member surface so the member can be placed in a single access opening of the planar wall with he light source and light detector outside the planar wall.