Image captioning, a fundamental task in vision-language understanding, seeks to generate accurate natural language descriptions for provided images. The CLIP model, with its rich semantic features learned from a large corpus of image-text pairs, is well-suited for this task. In this paper, we present a two-stage semi-supervised image captioning approach that exploits the potential of CLIP encoding. Our model comprises a CLIP visual encoder, a mapping network, and a language model for text generation. In the initial stage, we train the model using a small labeled dataset by contrasting the generated captions with the ground truth captions. In the subsequent stage, we continue the training using unlabeled images, aiming to maximize the image-caption similarity based on CLIP embeddings. Remarkably, despite utilizing less than 2% of the COCO-captions, our approach delivers a performance comparable to state-of-the-art models trained on the complete dataset. Furthermore, the captions generated by our approach are more distinctive, informative, and in line with human preference.

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