Apparatus for measuring the distance to a point on the inner wall of a hot furnace

A method for measuring the change in thickness between a first time and a second time of the lining on the inner wall of a furnace having a temperature of at least 500.degree. C., said method comprising the steps of:

• providing an electro-optical distance measuring instrument having the capability of transmitting a modulating radiation signal within a narrow frequency band, receiving the reflected transmitted signal, and determining by phase comparison methods the distance from the electro-optical measuring instrument to the point of reflection;
• providing an optical filter aligned only in the receiving path of said electro-optical distance measuring instrument to filter out all radiation other than said narrow frequency band of transmitted radiation, said optical filter comprising the combination of an interference filter and a Fabry-Perot filter wherein the central pass band of the Fabry-Perot filter lies within the pass band of the interference filter and the side bands of the Fabry-Perot filter lie outside the pass band of the interference filter;
• performing at the first time a first measurement of the distance from the electro-optical distance measuring instrument to a first point on the lining on the inner wall of the furnace, said first measurement being performed by:
• (i) focusing said modulated radiation signal of said electro-optical distance measuring instrument on said first point on the inner lining of the furnace;
• (ii) receiving the reflected transmitted signal from said first point through said optical filter;
• and
• (iii) determining by phase comparison methods the distance from the electro-optical distance measuring instrument to said first point on the inner wall of said furnace;
• determining the distance from said first point to a reference point fixed in relation to said furnace;
• performing at the second time a second measurement of the distance from the electro-optical distance measuring instrument to a second point on the lining on the inner wall of the furnace, said second point being in the vicinity of said first point, and said second measurement being performed by:
• (i) focusing said modulated radiation signal on said second point on the inner wall of said furnace;
• (ii) receiving the reflected transmitted signal from said second point through the optical filter;
• and
• (iii) determining by phase comparison methods the distance from the electro-optical distance measurement instrument to said second point;
• determining the distance from said second point to said reference point;
• and
• comparing said distance from said first point to said reference point with said distance from said second point to said reference point, and determining therefrom the change in thickness of said lining.