Ammonia is considered more and more as a promising carbon-free fuel for internal combustion engines to contribute to the decarbonization of several sectors where replacing conventional engines with batteries or fuel cells remains unsuitable. However, ammonia properties can induce some challenges for efficient and stable combustion. This study investigates the use of an active pre-chamber ignition system fueled with hydrogen and compares it to conventional spark ignition, with a focus on lean limit operation and early flame development. Experiments were conducted on a single cylinder optical engine with a compression ratio of 9.5, equipped with a quartz window in the piston for natural flame luminosity imaging using a high-speed camera. The engine was fueled with a mixture of 95% ammonia and 5% hydrogen by volume. Ammonia was injected and mixed with air in the intake port while hydrogen was directly injected into the prechamber. As a function of the intake pressure (1.0, 0.9, 0.8, and 0.7 bar), the lean operating limit was determined as 0.7, 0.8, 0.85, 0.9 for both active pre-chamber and spark ignition strategies. In fact, with hydrogen prechamber ignition, significant faster combustion was achieved which reduced by 30% the overall combustion duration. The visualization of natural emission of flame revealed a more distributed and faster flame propagation with the active prechamber, indicating a higher ignition performance and faster combustion under lean ammonia-hydrogen conditions, highlighting its potential for extending operating limits and improving combustion robustness.