Abstract: This paper is the second in a series of short tutorial articles loosely based on tutorial sessions given at the Royal Military College of Science to Masters of Science (MSc) students who are studying on defence technology courses. The purpose of the tutorials is to enable students to do first-pass (rough) calculations on various aspects of electro-optical systems (electro-optical system analysis). This type of electro-optical systems analysis is typical of that which they may carry out in their potential future role as defence analysts. This second paper looks at calculating the power received at a detector from a target that may be considered to be either a point source or an extended source. Such calculations enable the estimation of, for example, the lock-on range of an infrared homing missile. In any theatre of conflict or potential conflict the shear number of infrared homing missiles available is quite bewildering. These missiles range in complexity and cost from the simple hot-spot trackers such as SA7 to the complex fully imaging systems such as IR Maverick or the Advanced Short Range Air to Air Missile (ASRAAM). The success of these missiles is also very impressive with IR missile systems accounting for approximately 55% of worldwide combat aircraft losses in the past 20 years. It is necessary in some instances, for example in defence analysis, to make rough calculations on the feasibility of the operation of such systems in various conditions with little or practically no data available on the systems to be used. In some circumstances the data may simply be a report that an IR missile was used against a particular target and a poor quality photograph of the alleged missile. Under these circumstances it is therefore appropriate to undertake a process of very low-fidelity modelling to enable a decision of the probability of successful utilisation. This paper looks at such a low-fidelity model for IR homing missile systems.