A method for monitoring deposit build-up on spray nozzles (104, 301a,b) in a spray drying or spray cooling chamber, said method comprising: feeding drying air or cooling air into the chamber; feeding a feed material to be spray dried or spray cooled into the chamber through one or more spray nozzles (104, 301a, 301b), thereby obtaining one or more sprays of feed material in contact with drying or cooling air; and obtaining (402, 403) and recording images by use of one or more digital optical cameras (108a, 108b, 108c, 108d, 201, 203, 206), said images having an image area comprising a first area of interest covering at least part of a spray nozzle (104, 301a, 301b) and/or part of a feed material spray corresponding to a spray nozzle; said method being characterised in that: the step of obtaining and recording images (402, 403) comprises obtaining and recording a reference image by use of the one or more digital optical cameras (108a, 108b, 108c, 108d, 201, 203, 206), said reference image having an image area comprising the first area of interest; and repetitively obtaining and recording process images by use of the one or more digital optical cameras (108a, 108b, 108c, 108d, 201, 203, 206), said process images covering the same image area as the reference image; and in that the method comprises: comparing at least part of the recorded process images with the reference image and detecting (405, 501) differences within the first interest area between the reference image and each of a number of process images being compared to the reference image; and issuing a notification based on the detected differences. feeding drying air or cooling air into the chamber; feeding a feed material to be spray dried or spray cooled into the chamber through one or more spray nozzles (104, 301a, 301b), thereby obtaining one or more sprays of feed material in contact with drying or cooling air; and obtaining (402, 403) and recording images by use of one or more digital optical cameras (108a, 108b, 108c, 108d, 201, 203, 206), said images having an image area comprising a first area of interest covering at least part of a spray nozzle (104, 301a, 301b) and/or part of a feed material spray corresponding to a spray nozzle; said method being characterised in that: the step of obtaining and recording images (402, 403) comprises obtaining and recording a reference image by use of the one or more digital optical cameras (108a, 108b, 108c, 108d, 201, 203, 206), said reference image having an image area comprising the first area of interest; and repetitively obtaining and recording process images by use of the one or more digital optical cameras (108a, 108b, 108c, 108d, 201, 203, 206), said process images covering the same image area as the reference image; and in that the method comprises: comparing at least part of the recorded process images with the reference image and detecting (405, 501) differences within the first interest area between the reference image and each of a number of process images being compared to the reference image; and issuing a notification based on the detected differences. the step of obtaining and recording images (402, 403) comprises obtaining and recording a reference image by use of the one or more digital optical cameras (108a, 108b, 108c, 108d, 201, 203, 206), said reference image having an image area comprising the first area of interest; and repetitively obtaining and recording process images by use of the one or more digital optical cameras (108a, 108b, 108c, 108d, 201, 203, 206), said process images covering the same image area as the reference image; and in that the method comprises: comparing at least part of the recorded process images with the reference image and detecting (405, 501) differences within the first interest area between the reference image and each of a number of process images being compared to the reference image; and issuing a notification based on the detected differences. comparing at least part of the recorded process images with the reference image and detecting (405, 501) differences within the first interest area between the reference image and each of a number of process images being compared to the reference image; and issuing a notification based on the detected differences.