Final answer:
Gas masks and the short integration time of the human eye can cause security forces to not be able to see the contrast and details of intruders.
Step-by-step explanation:
One reason security forces may not be able to see contrast and details of intruders is the use of gas masks. While gas masks protect against chemical attacks, they can severely narrow the wearer's range of vision, making it difficult to see fine details. Another factor is the integration time of the human eye, which only takes a fraction of a second to add light energy together before sending the image to the brain. This short integration time limits the ability to perceive contrast and details in low-light or fast-moving situations.