Understanding fuel-air mixing inside the engine is essential for efficient combustion of fuel and reducing harmful emissions such as NOx and particulates in development of ultra-low emission diesel engines. Initial spray development and entrainment of surrounding air plays critical role in start of ignition in diesel engine. In the present study, spray development, entrainment and ignition in the early part of the diesel spray combustion is studied using ultra-high speed imaging inside a constant volume chamber. Elevated temperature and pressure conditions inside the constant volume chamber were achieved by pre-combustion of lean acetylene-air mixture. Acetylene and air were fed to the chamber precisely using control values and allowed to mix inside the chamber. Upon ignition with a spark plug, combustion of premixed gas mixture rises temperature and pressure inside the CVC. Pressure inside the CVC increases thereafter decreases due to cooling of combustion products and due to heat loss to chamber walls. Chamber pressure was monitored and fuel injection was set to occur when chamber pressure reaches certain value during cooling of combustion. Ambient conditions inside CVC at the time of injection were set at temperature of about 930 K and a pressure of about 2.55 MPa for the present study. Sprays were acquired from a 6-hole common rail diesel injector. A green LED was used to front illuminate the sprays, light scattered from sprays and the natural soot luminosity from combustion of sprays were recorded using a Photron FASTCAM SA-X2 high speed CMOS camera. Images were recorded at 135 kfps, 512 X 512 pixel2 with a pixel resolution of 130μm/pixel. The acquired images were analysed using matlab to characterise the effects of spray dispersion on ignition, entrainment and lift-off.