Table 2.

Effective factors influencing LIDAR intensity measurements.

Category	Factor	Description	Related References
Target Surface Characteristics	Reflectance (ρ)	By definition, surfaces of higher reflectance will reflect a greater portion of the incident laser radiation, thereby increasing the received signal power. In radiometric calibration, this is typically the parameter of interest.	[59,60,61,62,63,64,65]
	Roughness (ɳ)	Surface roughness dictates the type of reflection (e.g., specular vs. diffuse)	[62,66,67]
Acquisition Geometry	Range (R)	The emitted pulse energy decays as a function of range or distance traveled.	[27,58,63,64,65,68,69,70,71,72,73]
	Angle of Incidence (α)	Greater angles of incidence typically result in less of the incident laser energy being backscattered in the direction of the receiver, thereby reducing received optical power. Additionally, when the laser beam strikes a surface obliquely, it increases the backscattering cross section.	[58,62,63,64,65,66,68,69,70,71,72]
	Multiple Returns	When a single laser pulse reflects from objects, an attenuation correction can be applied to compensate for the energy split between objects.	[74,75,76]
Instrumental Effects	Transmitted Energy (E)	The amount of energy backscattered from targets is related to the amount of energy transmitted with every pulse. Transmitted pulse energy is related to peak transmitted power (which varies with pulse repetition frequency in many systems) and transmit pulse width.	[59,61,65,77]
	Intensity Bit Depth (*-bit) and Scaling	Different scanners use varying bit depth (e.g., 8-bit, 12-bit or 16-bit) when digitizing the return signal. Recorded digital numbers (DNs) are typically scaled to fill the available dynamic range.	[70,78]
	Amplifier for low reflective surfaces	Some scanners amplify the intensity values measured on low reflective surfaces.	[59,60,61,72]
	Automatic gain control (Ω)	Some systems (e.g., Leica ALS systems) employ automatic gain control (AGC), which increases the dynamic range that can be accommodated but can also result in discontinuities in the intensity signal, if not compensated.	[27,65,79]
	Brightness reducer for near distances	Some scanners reduce intensity values measured on close objects (e.g., less than 10 m distance).	[21,54,72]
	Aperture Size (Dr)	A larger aperture admits more light, increasing received signal strength.	[60]
Environmental Effects	Atmospheric Transmittance (T) or (ηatm)	Radiant energy attenuates in propagating through the atmosphere, as a function of humidity, temperature pressure and other variables.	[58,65,69,70]
	Wetness	Wet surfaces also absorb more energy from the pulse (particularly at the 1.5 micron wavelength used in some systems), resulting in weaker returns.	[61,69]