As deep neural networks (DNNs) are widely deployed in various applications, the security of pretrained DNNs is crucial since backdoors can be introduced through poisoned training. A backdoored DNN model works properly when benign inputs are provided, but it produces targeted misclassification on the inputs with an intended pattern known as a trojan trigger. Current technologies for trigger generation mainly focus on the physical and model domains. In this work, we investigate trojan triggers from the perception domain, especially the physical process of collecting light rays when they pass through the lens and hit the optical sensors. A new type of backdoor attack, Lens Flare attack, is introduced. It concentrates on the perception domain and is more physically plausible and stealthy. Experiments show that the DNNs with Lens Flare backdoor can achieve accuracy comparable to their original counterpart on benign input while misclassifying the input with high certainty if the Lens Flare trigger is present. It is also demonstrated that the Lens Flare backdoor is resistant to state-of-the-art backdoor defenses.