what is the "Surface spread exponent" in specular function?

I want to calculate reflectance using the function "specular". There is "Surface spread exponent". But I don't understand What is "Surface spread exponent"? So, I want to know how dose the "specular" function calculate the specular reflectance? Are there any formula or reference paper?

[Image Analyst]

Never heard of it. Do you mean Point Spread Function? Or does it have anything to do with the Bidirection reflectance distribution function?

[hyun-woo]

Yes, It is related to BRDF. but I can't know what is "Surface spread exponent" parameter in specular function

r = specular(nx,ny,nz,s,v,k)

k is "Surface spread exponent" parameter.

The "specular" function is in MATLAB -> Functions ->3-D Visualization -> Lighting