Abstract: A two-step method and apparatus for controlling cracking severity in the effluent from a cracking furnace such as an ethylene cracker. The method includes two steps. The first step consists of determining the near infrared spectrum of effluent in-line. The second step consists of changing the temperature and/or residence time of the furnace according to the determination of the first step. The apparatus includes a light source mounted on a conduit for the effluent, a light detector mounted on the opposite side of the conduit from the light source to receive light emitted from the light source, means for sheltering the light source from the effluent, means for sheltering the lights detector from the effluent, means for flowing a fluid past the light source at a higher pressure than the pressure of the effluent; and means for flowing a fluid past the light detector at a higher pressure than the pressure of the effluent.