Because a Jackson's chameleon's environment must stay within fairly narrow bounds, accurate monitoring is the foundation of every other piece of technology. Reliable digital thermometers and hygrometers tell you whether the basking zone, the ambient range, and the humidity are actually where you intend them to be, rather than where you assume they are.
The cheap analog dial gauges that often come bundled with enclosures are notoriously inaccurate and should be replaced with quality digital instruments. Probe-style sensors are especially useful because they let you measure conditions at specific points, such as right at the basking branch versus down in the cool zone, capturing the gradient that matters.
A temperature gun is a valuable complement to fixed sensors. Pointing an infrared thermometer directly at the basking surface gives you the actual temperature the animal experiences when it sits there, which a sensor mounted on the cage wall cannot tell you. This precision matters most for confirming the basking spot is warm enough without being dangerous.
Monitoring both extremes of the daily cycle is the goal. Knowing the peak daytime basking temperature and the nighttime low, along with how humidity rises and falls around misting, gives a complete picture of the environment and lets you catch a drift toward overheating or stagnation before it harms the animal.