Open access refers to the practice of making scholarly research freely available to the public online without restrictions on access. There are several models of open access publishing, each with its own characteristics and implications for authors, readers, and publishers. Here are the different types of open access options available:

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1. Immediate Open Access: In this model, articles are made freely available to readers immediately upon publication. Authors typically retain copyright to their work, and users are granted permission to access, download, distribute, and reuse the content without barriers. Immediate open access is often associated with journals that do not charge subscription fees and rely on other revenue sources, such as article processing charges (APCs) paid by authors.

2. Delayed Open Access: With delayed open access, articles are initially published behind a paywall or subscription barrier for a certain period (e.g., 6 months to 2 years) before becoming freely accessible to the public. This model allows publishers to recoup costs through subscriptions during the embargo period while still providing eventual access to the research without charge. After the embargo period expires, articles are typically made available under a Creative Commons license or similar open access terms.

3. Hybrid Models: Hybrid open access journals offer a combination of subscription-based and open access publishing options. Authors can choose to make their individual articles open access by paying an article processing charge (APC), while other articles in the same journal remain behind a paywall. Hybrid models allow authors to comply with funding mandates or preferences for open access while still publishing in established journals with high impact factors. However, they have faced criticism for double-dipping, where publishers receive revenue from both subscriptions and APCs.

4. Green Open Access (Self-Archiving): This model involves authors depositing preprints or postprints of their manuscripts in institutional or subject-based repositories, such as PubMed Central or arXiv, after an embargo period set by the publisher. The deposited versions are made freely available to the public, supplementing the publisher's version available through subscription or pay-per-view. Green open access provides an additional avenue for authors to share their research openly while retaining copyright and publishing in traditional subscription journals.

5. Gold Open Access: Gold open access journals make all articles freely available to readers immediately upon publication, without any subscription fees or access barriers. These journals typically rely on alternative funding models, such as article processing charges (APCs), membership fees, institutional subsidies, or philanthropic support, to cover publication costs. Gold open access journals often provide immediate visibility and accessibility to research findings, facilitating broader dissemination and impact.

6. Diamond/Open Diamond Open Access: Diamond open access, also known as community-driven open access or scholar-led open access, refers to journals that are entirely free to both authors and readers, with no fees for publication or access. These journals are often run by academic societies, research institutions, or volunteer-led initiatives and rely on community support, volunteer labor, or institutional funding to cover operating costs. Diamond open access journals prioritize scholarly communication and accessibility over commercial interests, promoting academic freedom and equity in publishing.

Each open access model has its advantages and challenges, and the choice of model may depend on factors such as funding availability, disciplinary norms, institutional policies, and author preferences. Ultimately, the goal of open access publishing is to maximize the dissemination and impact of scholarly research by making it freely accessible to everyone, regardless of geographic location, institutional affiliation, or financial resources.