The radiation includes a surface emission portion and a surface reflection portion which includes the probe beam energy reflected from the surface.

The subject (100) is illuminated with radiation,and radiation reflected from the subject is received.

Radiation reflected (90) from the tissue (71) is then collected to determine a reflection spectrum (step 18). pH is then determined by comparing this spectrum to a mathematical model relating optical properties to pH of the tissue (steps 20 and 22).

Intensity of the reflected radiation is measured at each illumination wavelength.

This minimizes the path of the reflected radiation.

The probe (116) receives reflected radiation from the tissue.

A transmitter (2) for transmitting radiation that can be reflected by the smoke (1) and a receiver (3) that detects the radiation reflected by the smoke are provided.

The detector is configured so that a ratio of the reflected radiation to the non-reflected radiation is higher in the first portion than the second portion.

The radiation detector (50) includes a beam splitter (56) and a pair of detector elements (52a, 52b) with polarizing filters (54a, 54b) which are oriented at 90 degrees relative to each other.

A detector is configured to measure radiation reflected from the substrate.

Information regarding the radiation reflected from the target is kept and information regarding the radiation reflected from the surface around the target is eliminated.

The consistency of the web (112) is measured by comparing the intensity of the microwave radiation (124) reflected from the web with the intensity of the radiation reflected in the reference measurement.

The radiation detector is located to receive reflected radiation from the reflective material along another axis.

The heat absorption device (3) receives solar radiation and at the same time receives reflected and concentrated radiation.

The portion of reflected radiation ER is minimized, and the transmitted radiation ET is increased accordingly.