fig1 is a plan view of an apparatus according to the invention is shown diagrammatically in which illumination is provided from an emitter 2 which is a laser having output 1 . 5 w , a beam width of about 2 mm , at a wavelength of 532 nm ( green ). the beam is fanned , using a cylindrical lens , across a duct 4 from one wall at approximately right angles to the beam wall , 50 % of the cross section of the duct being illuminated . at the opposite wall , a beam dump 6 is provided to absorb illumination received at that point so that essentially no emitted light is reflected back from that wall . sparkles from any particulate in the duct are detected by a 0 . 08 lux c / cs mount ccd camera 8 having a 12 mm lens having the centre of its field of view about at an angle of 45 ° across the duct . the camera is fitted with a zoom lens 10 to focus the field of view , and with a 532 nm filter 12 so as to accept only sparkles caused by light from the emitter , and with a graduated neutral density filter 14 ( c41 - 960 variable density beamsplitter , edmund optics ltd , york england ) to aid normalisation of sparkles occurring close to the camera 8 with those distant from the camera 8 . both the emitter 2 and the camera are located outside the wall of the duct 4 so that illumination and sparkle are transmitted through the duct wall through transparent windows ( not shown ). output from the camera 8 is then analysed using a laptop 16 so that required data on levels of particulate , e . g . identification and quantification of sparkles along the observed length , and any increase in levels with time can be calculated and monitored and any action re the particulate abatement system taken if required . fig2 is a 3 - dimensional diagram of a illumination beam from the laser described in example 1 in which the beam has been fanned using an optical lens across substantially the whole cross section of a duct ; the emitter was positioned at a corner of the duct . in a further embodiment of this apparatus , the beam from the laser is mechanically scanned using a stepper motor across substantially the whole cross section of the duct . fig3 and 4 show diagrammatically an apparatus according to the invention in which back scatter is detected using illumination from a laser emitter having 3 mw output at wavelength 680 nm . the apparatus is mounted at right angles to the direction of gas flow . the illumination from the laser emitter is fanned using an optical line generator which spreads ( i . e . fans ) the beam in one plane through 80 ° so that 60 % of the cross sectional area of the duct is illuminated . back scatter is detected using four phototransistors 18 arranged as a square around the emitter 2 . the output from the phototransistors is then amplified using high gain ac - coupled amplifiers 20 and summing amp 22 so that any effects of ambient light ( including varying changing ambient light ) are minimised , and so that only transient sparkles from passing particulate are captured . in this embodiment of the apparatus of the invention , the pc stores information received from the amplifiers , and then calculates the root mean square ( rms ) of the amplitudes of that information to produce a figure for of particulate concentration across the illuminated area . results of a number of rms calculations are compared in table 1 with “ alpha ” ( trade mark , unidata europe limited , sheffield , england ) readings ( a well established system which comprises a single path optical particulate beam monitor which records point to point measurements of particulate and then assumes that level uniformly over the whole area ). the results show that the apparatus is sensitive to passing particulate in the detectable range of the laser head . a large rms reading shows a good correspondence to the large alpha reading ( test 6 ); in respect of the low alpha readings , electrical and optical noise tended to distort the rms reading , but more sophisticated electronics would remedy same . however the apparatus of preferred embodiments of the present invention advantageously detects and monitors particulate over a substantial portion of the duct . fig5 shows diagrammatically a scanning arrangement for use in the apparatus of the invention . in this arrangement , an illumination beam from the laser 2 is directed to a mirror 24 located on a shaft of a gearbox 26 from a stepper motor 28 . the illumination from the mirror 24 may then be detected using detectors arranged as required . a “ home detector ” 25 may be used as a positional reference and for assisting in calculating any backlash in the gearbox 26 . a suitable stepper motor 28 has a step angle of 1 . 8 ° ( half step mode 0 . 9 °), step angle accuracy 5 %, voltage 5v ; rated current 0 . 5 a / phase . a suitable gearbox 26 scans essentially the entire cross section of a duct ; it is also has a ratio of 100 : 1 ( step angle 0 . 0180 / 0 . 0090 ). such a scanning arrangement is robust , vibration proof and capable being used over a wide temperature range , has low scan speed , repeatable positional information , and is easy to maintain . in this example , a series of experiments was carried out in one gantry of a filter house of a gas turbine electricity generating station , in which the apparatus of the invention was located in a duct upstream of the turbine in the gas inlet to the turbine . the gantry was about 15 m long and about 1 m wide . the apparatus was set up adjacent an array of filters in the gantry . an apparatus similar to that in fig3 and example 3 except that the laser emitter had power between 200 mw and 1 . 6 w at a wavelength of 532 nm . the variable power allowed for rapid evaluation of the performance of the system over a wide range of optical power levels . the detector was a photodiode provided with a 35 mm - 70 mm zoom lens , an optical band pass filter tuned to 532 nm , signal conditioners and amplifiers so that the signal could be fed to a digital oscilloscope and pc for storage of data . the detector system used is shown diagrammatically in fig4 . tests were carried out at various laser power outputs . background signals were taken ( i . e . with only ambient particulate present ) and after introduction of particulate by sprinkling particulate into the monitored portion of the laser emitter beam . fig6 a and 6 b are oscilloscope traces at a laser power of 1 . 6 w , bias 5 μa , gain 23 from with background particulate ( 0 . 22 mg / m 3 ) and after introduction of particulate into the duct ( 9 . 74 mg / m 3 ) respectively . fig7 a and 7 b are oscilloscope traces at a laser power of 500 mw , bias 5 μa , gain 48 from with background particulate ( 0 . 22 mg / m 3 ) and after introduction of particulate ( 11 . 2 mg / m 3 ) respectively . from fig6 b and fig7 b it can be seen that the presence of particulate is readily detected .